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3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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25 
26 package java.util;
27 import java.io.Serializable;
28 import java.io.ObjectInputStream;
29 import java.io.ObjectOutputStream;
30 import java.io.IOException;
31 import java.lang.reflect.Array;
32 import java.util.function.BiConsumer;
33 import java.util.function.BiFunction;
34 import java.util.function.Consumer;
35 import java.util.function.Function;
36 import java.util.function.Predicate;
37 import java.util.function.UnaryOperator;
38 import java.util.stream.IntStream;
39 import java.util.stream.Stream;
40 import java.util.stream.StreamSupport;
41 import sun.misc.SharedSecrets;
42 
43 /**
44  * This class consists exclusively of static methods that operate on or return
45  * collections.  It contains polymorphic algorithms that operate on
46  * collections, "wrappers", which return a new collection backed by a
47  * specified collection, and a few other odds and ends.
48  *
49  * <p>The methods of this class all throw a <tt>NullPointerException</tt>
50  * if the collections or class objects provided to them are null.
51  *
52  * <p>The documentation for the polymorphic algorithms contained in this class
53  * generally includes a brief description of the <i>implementation</i>.  Such
54  * descriptions should be regarded as <i>implementation notes</i>, rather than
55  * parts of the <i>specification</i>.  Implementors should feel free to
56  * substitute other algorithms, so long as the specification itself is adhered
57  * to.  (For example, the algorithm used by <tt>sort</tt> does not have to be
58  * a mergesort, but it does have to be <i>stable</i>.)
59  *
60  * <p>The "destructive" algorithms contained in this class, that is, the
61  * algorithms that modify the collection on which they operate, are specified
62  * to throw <tt>UnsupportedOperationException</tt> if the collection does not
63  * support the appropriate mutation primitive(s), such as the <tt>set</tt>
64  * method.  These algorithms may, but are not required to, throw this
65  * exception if an invocation would have no effect on the collection.  For
66  * example, invoking the <tt>sort</tt> method on an unmodifiable list that is
67  * already sorted may or may not throw <tt>UnsupportedOperationException</tt>.
68  *
69  * <p>This class is a member of the
70  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
71  * Java Collections Framework</a>.
72  *
73  * @author  Josh Bloch
74  * @author  Neal Gafter
75  * @see     Collection
76  * @see     Set
77  * @see     List
78  * @see     Map
79  * @since   1.2
80  */
81 
82 public class Collections {
83     // Suppresses default constructor, ensuring non-instantiability.
Collections()84     private Collections() {
85     }
86 
87     // Algorithms
88 
89     /*
90      * Tuning parameters for algorithms - Many of the List algorithms have
91      * two implementations, one of which is appropriate for RandomAccess
92      * lists, the other for "sequential."  Often, the random access variant
93      * yields better performance on small sequential access lists.  The
94      * tuning parameters below determine the cutoff point for what constitutes
95      * a "small" sequential access list for each algorithm.  The values below
96      * were empirically determined to work well for LinkedList. Hopefully
97      * they should be reasonable for other sequential access List
98      * implementations.  Those doing performance work on this code would
99      * do well to validate the values of these parameters from time to time.
100      * (The first word of each tuning parameter name is the algorithm to which
101      * it applies.)
102      */
103     private static final int BINARYSEARCH_THRESHOLD   = 5000;
104     private static final int REVERSE_THRESHOLD        =   18;
105     private static final int SHUFFLE_THRESHOLD        =    5;
106     private static final int FILL_THRESHOLD           =   25;
107     private static final int ROTATE_THRESHOLD         =  100;
108     private static final int COPY_THRESHOLD           =   10;
109     private static final int REPLACEALL_THRESHOLD     =   11;
110     private static final int INDEXOFSUBLIST_THRESHOLD =   35;
111 
112     /**
113      * Sorts the specified list into ascending order, according to the
114      * {@linkplain Comparable natural ordering} of its elements.
115      * All elements in the list must implement the {@link Comparable}
116      * interface.  Furthermore, all elements in the list must be
117      * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}
118      * must not throw a {@code ClassCastException} for any elements
119      * {@code e1} and {@code e2} in the list).
120      *
121      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
122      * not be reordered as a result of the sort.
123      *
124      * <p>The specified list must be modifiable, but need not be resizable.
125      *
126      * @implNote
127      * This implementation defers to the {@link List#sort(Comparator)}
128      * method using the specified list and a {@code null} comparator.
129      *
130      * @param  <T> the class of the objects in the list
131      * @param  list the list to be sorted.
132      * @throws ClassCastException if the list contains elements that are not
133      *         <i>mutually comparable</i> (for example, strings and integers).
134      * @throws UnsupportedOperationException if the specified list's
135      *         list-iterator does not support the {@code set} operation.
136      * @throws IllegalArgumentException (optional) if the implementation
137      *         detects that the natural ordering of the list elements is
138      *         found to violate the {@link Comparable} contract
139      * @see List#sort(Comparator)
140      */
141     @SuppressWarnings("unchecked")
sort(List<T> list)142     public static <T extends Comparable<? super T>> void sort(List<T> list) {
143         list.sort(null);
144     }
145 
146     /**
147      * Sorts the specified list according to the order induced by the
148      * specified comparator.  All elements in the list must be <i>mutually
149      * comparable</i> using the specified comparator (that is,
150      * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
151      * for any elements {@code e1} and {@code e2} in the list).
152      *
153      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
154      * not be reordered as a result of the sort.
155      *
156      * <p>The specified list must be modifiable, but need not be resizable.
157      *
158      * @implNote
159      * This implementation defers to the {@link List#sort(Comparator)}
160      * method using the specified list and comparator.
161      *
162      * @param  <T> the class of the objects in the list
163      * @param  list the list to be sorted.
164      * @param  c the comparator to determine the order of the list.  A
165      *        {@code null} value indicates that the elements' <i>natural
166      *        ordering</i> should be used.
167      * @throws ClassCastException if the list contains elements that are not
168      *         <i>mutually comparable</i> using the specified comparator.
169      * @throws UnsupportedOperationException if the specified list's
170      *         list-iterator does not support the {@code set} operation.
171      * @throws IllegalArgumentException (optional) if the comparator is
172      *         found to violate the {@link Comparator} contract
173      * @see List#sort(Comparator)
174      */
175     @SuppressWarnings({"unchecked", "rawtypes"})
sort(List<T> list, Comparator<? super T> c)176     public static <T> void sort(List<T> list, Comparator<? super T> c) {
177         list.sort(c);
178     }
179 
180 
181     /**
182      * Searches the specified list for the specified object using the binary
183      * search algorithm.  The list must be sorted into ascending order
184      * according to the {@linkplain Comparable natural ordering} of its
185      * elements (as by the {@link #sort(List)} method) prior to making this
186      * call.  If it is not sorted, the results are undefined.  If the list
187      * contains multiple elements equal to the specified object, there is no
188      * guarantee which one will be found.
189      *
190      * <p>This method runs in log(n) time for a "random access" list (which
191      * provides near-constant-time positional access).  If the specified list
192      * does not implement the {@link RandomAccess} interface and is large,
193      * this method will do an iterator-based binary search that performs
194      * O(n) link traversals and O(log n) element comparisons.
195      *
196      * @param  <T> the class of the objects in the list
197      * @param  list the list to be searched.
198      * @param  key the key to be searched for.
199      * @return the index of the search key, if it is contained in the list;
200      *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The
201      *         <i>insertion point</i> is defined as the point at which the
202      *         key would be inserted into the list: the index of the first
203      *         element greater than the key, or <tt>list.size()</tt> if all
204      *         elements in the list are less than the specified key.  Note
205      *         that this guarantees that the return value will be &gt;= 0 if
206      *         and only if the key is found.
207      * @throws ClassCastException if the list contains elements that are not
208      *         <i>mutually comparable</i> (for example, strings and
209      *         integers), or the search key is not mutually comparable
210      *         with the elements of the list.
211      */
212     public static <T>
binarySearch(List<? extends Comparable<? super T>> list, T key)213     int binarySearch(List<? extends Comparable<? super T>> list, T key) {
214         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
215             return Collections.indexedBinarySearch(list, key);
216         else
217             return Collections.iteratorBinarySearch(list, key);
218     }
219 
220     private static <T>
indexedBinarySearch(List<? extends Comparable<? super T>> list, T key)221     int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
222         int low = 0;
223         int high = list.size()-1;
224 
225         while (low <= high) {
226             int mid = (low + high) >>> 1;
227             Comparable<? super T> midVal = list.get(mid);
228             int cmp = midVal.compareTo(key);
229 
230             if (cmp < 0)
231                 low = mid + 1;
232             else if (cmp > 0)
233                 high = mid - 1;
234             else
235                 return mid; // key found
236         }
237         return -(low + 1);  // key not found
238     }
239 
240     private static <T>
iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)241     int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)
242     {
243         int low = 0;
244         int high = list.size()-1;
245         ListIterator<? extends Comparable<? super T>> i = list.listIterator();
246 
247         while (low <= high) {
248             int mid = (low + high) >>> 1;
249             Comparable<? super T> midVal = get(i, mid);
250             int cmp = midVal.compareTo(key);
251 
252             if (cmp < 0)
253                 low = mid + 1;
254             else if (cmp > 0)
255                 high = mid - 1;
256             else
257                 return mid; // key found
258         }
259         return -(low + 1);  // key not found
260     }
261 
262     /**
263      * Gets the ith element from the given list by repositioning the specified
264      * list listIterator.
265      */
get(ListIterator<? extends T> i, int index)266     private static <T> T get(ListIterator<? extends T> i, int index) {
267         T obj = null;
268         int pos = i.nextIndex();
269         if (pos <= index) {
270             do {
271                 obj = i.next();
272             } while (pos++ < index);
273         } else {
274             do {
275                 obj = i.previous();
276             } while (--pos > index);
277         }
278         return obj;
279     }
280 
281     /**
282      * Searches the specified list for the specified object using the binary
283      * search algorithm.  The list must be sorted into ascending order
284      * according to the specified comparator (as by the
285      * {@link #sort(List, Comparator) sort(List, Comparator)}
286      * method), prior to making this call.  If it is
287      * not sorted, the results are undefined.  If the list contains multiple
288      * elements equal to the specified object, there is no guarantee which one
289      * will be found.
290      *
291      * <p>This method runs in log(n) time for a "random access" list (which
292      * provides near-constant-time positional access).  If the specified list
293      * does not implement the {@link RandomAccess} interface and is large,
294      * this method will do an iterator-based binary search that performs
295      * O(n) link traversals and O(log n) element comparisons.
296      *
297      * @param  <T> the class of the objects in the list
298      * @param  list the list to be searched.
299      * @param  key the key to be searched for.
300      * @param  c the comparator by which the list is ordered.
301      *         A <tt>null</tt> value indicates that the elements'
302      *         {@linkplain Comparable natural ordering} should be used.
303      * @return the index of the search key, if it is contained in the list;
304      *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The
305      *         <i>insertion point</i> is defined as the point at which the
306      *         key would be inserted into the list: the index of the first
307      *         element greater than the key, or <tt>list.size()</tt> if all
308      *         elements in the list are less than the specified key.  Note
309      *         that this guarantees that the return value will be &gt;= 0 if
310      *         and only if the key is found.
311      * @throws ClassCastException if the list contains elements that are not
312      *         <i>mutually comparable</i> using the specified comparator,
313      *         or the search key is not mutually comparable with the
314      *         elements of the list using this comparator.
315      */
316     @SuppressWarnings("unchecked")
binarySearch(List<? extends T> list, T key, Comparator<? super T> c)317     public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
318         if (c==null)
319             return binarySearch((List<? extends Comparable<? super T>>) list, key);
320 
321         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
322             return Collections.indexedBinarySearch(list, key, c);
323         else
324             return Collections.iteratorBinarySearch(list, key, c);
325     }
326 
indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c)327     private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
328         int low = 0;
329         int high = l.size()-1;
330 
331         while (low <= high) {
332             int mid = (low + high) >>> 1;
333             T midVal = l.get(mid);
334             int cmp = c.compare(midVal, key);
335 
336             if (cmp < 0)
337                 low = mid + 1;
338             else if (cmp > 0)
339                 high = mid - 1;
340             else
341                 return mid; // key found
342         }
343         return -(low + 1);  // key not found
344     }
345 
iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c)346     private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
347         int low = 0;
348         int high = l.size()-1;
349         ListIterator<? extends T> i = l.listIterator();
350 
351         while (low <= high) {
352             int mid = (low + high) >>> 1;
353             T midVal = get(i, mid);
354             int cmp = c.compare(midVal, key);
355 
356             if (cmp < 0)
357                 low = mid + 1;
358             else if (cmp > 0)
359                 high = mid - 1;
360             else
361                 return mid; // key found
362         }
363         return -(low + 1);  // key not found
364     }
365 
366     /**
367      * Reverses the order of the elements in the specified list.<p>
368      *
369      * This method runs in linear time.
370      *
371      * @param  list the list whose elements are to be reversed.
372      * @throws UnsupportedOperationException if the specified list or
373      *         its list-iterator does not support the <tt>set</tt> operation.
374      */
375     @SuppressWarnings({"rawtypes", "unchecked"})
reverse(List<?> list)376     public static void reverse(List<?> list) {
377         int size = list.size();
378         if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
379             for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
380                 swap(list, i, j);
381         } else {
382             // instead of using a raw type here, it's possible to capture
383             // the wildcard but it will require a call to a supplementary
384             // private method
385             ListIterator fwd = list.listIterator();
386             ListIterator rev = list.listIterator(size);
387             for (int i=0, mid=list.size()>>1; i<mid; i++) {
388                 Object tmp = fwd.next();
389                 fwd.set(rev.previous());
390                 rev.set(tmp);
391             }
392         }
393     }
394 
395     /**
396      * Randomly permutes the specified list using a default source of
397      * randomness.  All permutations occur with approximately equal
398      * likelihood.
399      *
400      * <p>The hedge "approximately" is used in the foregoing description because
401      * default source of randomness is only approximately an unbiased source
402      * of independently chosen bits. If it were a perfect source of randomly
403      * chosen bits, then the algorithm would choose permutations with perfect
404      * uniformity.
405      *
406      * <p>This implementation traverses the list backwards, from the last
407      * element up to the second, repeatedly swapping a randomly selected element
408      * into the "current position".  Elements are randomly selected from the
409      * portion of the list that runs from the first element to the current
410      * position, inclusive.
411      *
412      * <p>This method runs in linear time.  If the specified list does not
413      * implement the {@link RandomAccess} interface and is large, this
414      * implementation dumps the specified list into an array before shuffling
415      * it, and dumps the shuffled array back into the list.  This avoids the
416      * quadratic behavior that would result from shuffling a "sequential
417      * access" list in place.
418      *
419      * @param  list the list to be shuffled.
420      * @throws UnsupportedOperationException if the specified list or
421      *         its list-iterator does not support the <tt>set</tt> operation.
422      */
shuffle(List<?> list)423     public static void shuffle(List<?> list) {
424         Random rnd = r;
425         if (rnd == null)
426             r = rnd = new Random(); // harmless race.
427         shuffle(list, rnd);
428     }
429 
430     private static Random r;
431 
432     /**
433      * Randomly permute the specified list using the specified source of
434      * randomness.  All permutations occur with equal likelihood
435      * assuming that the source of randomness is fair.<p>
436      *
437      * This implementation traverses the list backwards, from the last element
438      * up to the second, repeatedly swapping a randomly selected element into
439      * the "current position".  Elements are randomly selected from the
440      * portion of the list that runs from the first element to the current
441      * position, inclusive.<p>
442      *
443      * This method runs in linear time.  If the specified list does not
444      * implement the {@link RandomAccess} interface and is large, this
445      * implementation dumps the specified list into an array before shuffling
446      * it, and dumps the shuffled array back into the list.  This avoids the
447      * quadratic behavior that would result from shuffling a "sequential
448      * access" list in place.
449      *
450      * @param  list the list to be shuffled.
451      * @param  rnd the source of randomness to use to shuffle the list.
452      * @throws UnsupportedOperationException if the specified list or its
453      *         list-iterator does not support the <tt>set</tt> operation.
454      */
455     @SuppressWarnings({"rawtypes", "unchecked"})
shuffle(List<?> list, Random rnd)456     public static void shuffle(List<?> list, Random rnd) {
457         int size = list.size();
458         if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
459             for (int i=size; i>1; i--)
460                 swap(list, i-1, rnd.nextInt(i));
461         } else {
462             Object[] arr = list.toArray();
463 
464             // Shuffle array
465             for (int i=size; i>1; i--)
466                 swap(arr, i-1, rnd.nextInt(i));
467 
468             // Dump array back into list
469             // instead of using a raw type here, it's possible to capture
470             // the wildcard but it will require a call to a supplementary
471             // private method
472             ListIterator it = list.listIterator();
473             for (int i=0; i<arr.length; i++) {
474                 it.next();
475                 it.set(arr[i]);
476             }
477         }
478     }
479 
480     /**
481      * Swaps the elements at the specified positions in the specified list.
482      * (If the specified positions are equal, invoking this method leaves
483      * the list unchanged.)
484      *
485      * @param list The list in which to swap elements.
486      * @param i the index of one element to be swapped.
487      * @param j the index of the other element to be swapped.
488      * @throws IndexOutOfBoundsException if either <tt>i</tt> or <tt>j</tt>
489      *         is out of range (i &lt; 0 || i &gt;= list.size()
490      *         || j &lt; 0 || j &gt;= list.size()).
491      * @since 1.4
492      */
493     @SuppressWarnings({"rawtypes", "unchecked"})
swap(List<?> list, int i, int j)494     public static void swap(List<?> list, int i, int j) {
495         // instead of using a raw type here, it's possible to capture
496         // the wildcard but it will require a call to a supplementary
497         // private method
498         final List l = list;
499         l.set(i, l.set(j, l.get(i)));
500     }
501 
502     /**
503      * Swaps the two specified elements in the specified array.
504      */
swap(Object[] arr, int i, int j)505     private static void swap(Object[] arr, int i, int j) {
506         Object tmp = arr[i];
507         arr[i] = arr[j];
508         arr[j] = tmp;
509     }
510 
511     /**
512      * Replaces all of the elements of the specified list with the specified
513      * element. <p>
514      *
515      * This method runs in linear time.
516      *
517      * @param  <T> the class of the objects in the list
518      * @param  list the list to be filled with the specified element.
519      * @param  obj The element with which to fill the specified list.
520      * @throws UnsupportedOperationException if the specified list or its
521      *         list-iterator does not support the <tt>set</tt> operation.
522      */
fill(List<? super T> list, T obj)523     public static <T> void fill(List<? super T> list, T obj) {
524         int size = list.size();
525 
526         if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
527             for (int i=0; i<size; i++)
528                 list.set(i, obj);
529         } else {
530             ListIterator<? super T> itr = list.listIterator();
531             for (int i=0; i<size; i++) {
532                 itr.next();
533                 itr.set(obj);
534             }
535         }
536     }
537 
538     /**
539      * Copies all of the elements from one list into another.  After the
540      * operation, the index of each copied element in the destination list
541      * will be identical to its index in the source list.  The destination
542      * list must be at least as long as the source list.  If it is longer, the
543      * remaining elements in the destination list are unaffected. <p>
544      *
545      * This method runs in linear time.
546      *
547      * @param  <T> the class of the objects in the lists
548      * @param  dest The destination list.
549      * @param  src The source list.
550      * @throws IndexOutOfBoundsException if the destination list is too small
551      *         to contain the entire source List.
552      * @throws UnsupportedOperationException if the destination list's
553      *         list-iterator does not support the <tt>set</tt> operation.
554      */
copy(List<? super T> dest, List<? extends T> src)555     public static <T> void copy(List<? super T> dest, List<? extends T> src) {
556         int srcSize = src.size();
557         if (srcSize > dest.size())
558             throw new IndexOutOfBoundsException("Source does not fit in dest");
559 
560         if (srcSize < COPY_THRESHOLD ||
561             (src instanceof RandomAccess && dest instanceof RandomAccess)) {
562             for (int i=0; i<srcSize; i++)
563                 dest.set(i, src.get(i));
564         } else {
565             ListIterator<? super T> di=dest.listIterator();
566             ListIterator<? extends T> si=src.listIterator();
567             for (int i=0; i<srcSize; i++) {
568                 di.next();
569                 di.set(si.next());
570             }
571         }
572     }
573 
574     /**
575      * Returns the minimum element of the given collection, according to the
576      * <i>natural ordering</i> of its elements.  All elements in the
577      * collection must implement the <tt>Comparable</tt> interface.
578      * Furthermore, all elements in the collection must be <i>mutually
579      * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
580      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
581      * <tt>e2</tt> in the collection).<p>
582      *
583      * This method iterates over the entire collection, hence it requires
584      * time proportional to the size of the collection.
585      *
586      * @param  <T> the class of the objects in the collection
587      * @param  coll the collection whose minimum element is to be determined.
588      * @return the minimum element of the given collection, according
589      *         to the <i>natural ordering</i> of its elements.
590      * @throws ClassCastException if the collection contains elements that are
591      *         not <i>mutually comparable</i> (for example, strings and
592      *         integers).
593      * @throws NoSuchElementException if the collection is empty.
594      * @see Comparable
595      */
min(Collection<? extends T> coll)596     public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
597         Iterator<? extends T> i = coll.iterator();
598         T candidate = i.next();
599 
600         while (i.hasNext()) {
601             T next = i.next();
602             if (next.compareTo(candidate) < 0)
603                 candidate = next;
604         }
605         return candidate;
606     }
607 
608     /**
609      * Returns the minimum element of the given collection, according to the
610      * order induced by the specified comparator.  All elements in the
611      * collection must be <i>mutually comparable</i> by the specified
612      * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
613      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
614      * <tt>e2</tt> in the collection).<p>
615      *
616      * This method iterates over the entire collection, hence it requires
617      * time proportional to the size of the collection.
618      *
619      * @param  <T> the class of the objects in the collection
620      * @param  coll the collection whose minimum element is to be determined.
621      * @param  comp the comparator with which to determine the minimum element.
622      *         A <tt>null</tt> value indicates that the elements' <i>natural
623      *         ordering</i> should be used.
624      * @return the minimum element of the given collection, according
625      *         to the specified comparator.
626      * @throws ClassCastException if the collection contains elements that are
627      *         not <i>mutually comparable</i> using the specified comparator.
628      * @throws NoSuchElementException if the collection is empty.
629      * @see Comparable
630      */
631     @SuppressWarnings({"unchecked", "rawtypes"})
min(Collection<? extends T> coll, Comparator<? super T> comp)632     public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
633         if (comp==null)
634             return (T)min((Collection) coll);
635 
636         Iterator<? extends T> i = coll.iterator();
637         T candidate = i.next();
638 
639         while (i.hasNext()) {
640             T next = i.next();
641             if (comp.compare(next, candidate) < 0)
642                 candidate = next;
643         }
644         return candidate;
645     }
646 
647     /**
648      * Returns the maximum element of the given collection, according to the
649      * <i>natural ordering</i> of its elements.  All elements in the
650      * collection must implement the <tt>Comparable</tt> interface.
651      * Furthermore, all elements in the collection must be <i>mutually
652      * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
653      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
654      * <tt>e2</tt> in the collection).<p>
655      *
656      * This method iterates over the entire collection, hence it requires
657      * time proportional to the size of the collection.
658      *
659      * @param  <T> the class of the objects in the collection
660      * @param  coll the collection whose maximum element is to be determined.
661      * @return the maximum element of the given collection, according
662      *         to the <i>natural ordering</i> of its elements.
663      * @throws ClassCastException if the collection contains elements that are
664      *         not <i>mutually comparable</i> (for example, strings and
665      *         integers).
666      * @throws NoSuchElementException if the collection is empty.
667      * @see Comparable
668      */
max(Collection<? extends T> coll)669     public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {
670         Iterator<? extends T> i = coll.iterator();
671         T candidate = i.next();
672 
673         while (i.hasNext()) {
674             T next = i.next();
675             if (next.compareTo(candidate) > 0)
676                 candidate = next;
677         }
678         return candidate;
679     }
680 
681     /**
682      * Returns the maximum element of the given collection, according to the
683      * order induced by the specified comparator.  All elements in the
684      * collection must be <i>mutually comparable</i> by the specified
685      * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
686      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
687      * <tt>e2</tt> in the collection).<p>
688      *
689      * This method iterates over the entire collection, hence it requires
690      * time proportional to the size of the collection.
691      *
692      * @param  <T> the class of the objects in the collection
693      * @param  coll the collection whose maximum element is to be determined.
694      * @param  comp the comparator with which to determine the maximum element.
695      *         A <tt>null</tt> value indicates that the elements' <i>natural
696      *        ordering</i> should be used.
697      * @return the maximum element of the given collection, according
698      *         to the specified comparator.
699      * @throws ClassCastException if the collection contains elements that are
700      *         not <i>mutually comparable</i> using the specified comparator.
701      * @throws NoSuchElementException if the collection is empty.
702      * @see Comparable
703      */
704     @SuppressWarnings({"unchecked", "rawtypes"})
max(Collection<? extends T> coll, Comparator<? super T> comp)705     public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
706         if (comp==null)
707             return (T)max((Collection) coll);
708 
709         Iterator<? extends T> i = coll.iterator();
710         T candidate = i.next();
711 
712         while (i.hasNext()) {
713             T next = i.next();
714             if (comp.compare(next, candidate) > 0)
715                 candidate = next;
716         }
717         return candidate;
718     }
719 
720     /**
721      * Rotates the elements in the specified list by the specified distance.
722      * After calling this method, the element at index <tt>i</tt> will be
723      * the element previously at index <tt>(i - distance)</tt> mod
724      * <tt>list.size()</tt>, for all values of <tt>i</tt> between <tt>0</tt>
725      * and <tt>list.size()-1</tt>, inclusive.  (This method has no effect on
726      * the size of the list.)
727      *
728      * <p>For example, suppose <tt>list</tt> comprises<tt> [t, a, n, k, s]</tt>.
729      * After invoking <tt>Collections.rotate(list, 1)</tt> (or
730      * <tt>Collections.rotate(list, -4)</tt>), <tt>list</tt> will comprise
731      * <tt>[s, t, a, n, k]</tt>.
732      *
733      * <p>Note that this method can usefully be applied to sublists to
734      * move one or more elements within a list while preserving the
735      * order of the remaining elements.  For example, the following idiom
736      * moves the element at index <tt>j</tt> forward to position
737      * <tt>k</tt> (which must be greater than or equal to <tt>j</tt>):
738      * <pre>
739      *     Collections.rotate(list.subList(j, k+1), -1);
740      * </pre>
741      * To make this concrete, suppose <tt>list</tt> comprises
742      * <tt>[a, b, c, d, e]</tt>.  To move the element at index <tt>1</tt>
743      * (<tt>b</tt>) forward two positions, perform the following invocation:
744      * <pre>
745      *     Collections.rotate(l.subList(1, 4), -1);
746      * </pre>
747      * The resulting list is <tt>[a, c, d, b, e]</tt>.
748      *
749      * <p>To move more than one element forward, increase the absolute value
750      * of the rotation distance.  To move elements backward, use a positive
751      * shift distance.
752      *
753      * <p>If the specified list is small or implements the {@link
754      * RandomAccess} interface, this implementation exchanges the first
755      * element into the location it should go, and then repeatedly exchanges
756      * the displaced element into the location it should go until a displaced
757      * element is swapped into the first element.  If necessary, the process
758      * is repeated on the second and successive elements, until the rotation
759      * is complete.  If the specified list is large and doesn't implement the
760      * <tt>RandomAccess</tt> interface, this implementation breaks the
761      * list into two sublist views around index <tt>-distance mod size</tt>.
762      * Then the {@link #reverse(List)} method is invoked on each sublist view,
763      * and finally it is invoked on the entire list.  For a more complete
764      * description of both algorithms, see Section 2.3 of Jon Bentley's
765      * <i>Programming Pearls</i> (Addison-Wesley, 1986).
766      *
767      * @param list the list to be rotated.
768      * @param distance the distance to rotate the list.  There are no
769      *        constraints on this value; it may be zero, negative, or
770      *        greater than <tt>list.size()</tt>.
771      * @throws UnsupportedOperationException if the specified list or
772      *         its list-iterator does not support the <tt>set</tt> operation.
773      * @since 1.4
774      */
rotate(List<?> list, int distance)775     public static void rotate(List<?> list, int distance) {
776         if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD)
777             rotate1(list, distance);
778         else
779             rotate2(list, distance);
780     }
781 
rotate1(List<T> list, int distance)782     private static <T> void rotate1(List<T> list, int distance) {
783         int size = list.size();
784         if (size == 0)
785             return;
786         distance = distance % size;
787         if (distance < 0)
788             distance += size;
789         if (distance == 0)
790             return;
791 
792         for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
793             T displaced = list.get(cycleStart);
794             int i = cycleStart;
795             do {
796                 i += distance;
797                 if (i >= size)
798                     i -= size;
799                 displaced = list.set(i, displaced);
800                 nMoved ++;
801             } while (i != cycleStart);
802         }
803     }
804 
rotate2(List<?> list, int distance)805     private static void rotate2(List<?> list, int distance) {
806         int size = list.size();
807         if (size == 0)
808             return;
809         int mid =  -distance % size;
810         if (mid < 0)
811             mid += size;
812         if (mid == 0)
813             return;
814 
815         reverse(list.subList(0, mid));
816         reverse(list.subList(mid, size));
817         reverse(list);
818     }
819 
820     /**
821      * Replaces all occurrences of one specified value in a list with another.
822      * More formally, replaces with <tt>newVal</tt> each element <tt>e</tt>
823      * in <tt>list</tt> such that
824      * <tt>(oldVal==null ? e==null : oldVal.equals(e))</tt>.
825      * (This method has no effect on the size of the list.)
826      *
827      * @param  <T> the class of the objects in the list
828      * @param list the list in which replacement is to occur.
829      * @param oldVal the old value to be replaced.
830      * @param newVal the new value with which <tt>oldVal</tt> is to be
831      *        replaced.
832      * @return <tt>true</tt> if <tt>list</tt> contained one or more elements
833      *         <tt>e</tt> such that
834      *         <tt>(oldVal==null ?  e==null : oldVal.equals(e))</tt>.
835      * @throws UnsupportedOperationException if the specified list or
836      *         its list-iterator does not support the <tt>set</tt> operation.
837      * @since  1.4
838      */
replaceAll(List<T> list, T oldVal, T newVal)839     public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
840         boolean result = false;
841         int size = list.size();
842         if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
843             if (oldVal==null) {
844                 for (int i=0; i<size; i++) {
845                     if (list.get(i)==null) {
846                         list.set(i, newVal);
847                         result = true;
848                     }
849                 }
850             } else {
851                 for (int i=0; i<size; i++) {
852                     if (oldVal.equals(list.get(i))) {
853                         list.set(i, newVal);
854                         result = true;
855                     }
856                 }
857             }
858         } else {
859             ListIterator<T> itr=list.listIterator();
860             if (oldVal==null) {
861                 for (int i=0; i<size; i++) {
862                     if (itr.next()==null) {
863                         itr.set(newVal);
864                         result = true;
865                     }
866                 }
867             } else {
868                 for (int i=0; i<size; i++) {
869                     if (oldVal.equals(itr.next())) {
870                         itr.set(newVal);
871                         result = true;
872                     }
873                 }
874             }
875         }
876         return result;
877     }
878 
879     /**
880      * Returns the starting position of the first occurrence of the specified
881      * target list within the specified source list, or -1 if there is no
882      * such occurrence.  More formally, returns the lowest index <tt>i</tt>
883      * such that {@code source.subList(i, i+target.size()).equals(target)},
884      * or -1 if there is no such index.  (Returns -1 if
885      * {@code target.size() > source.size()})
886      *
887      * <p>This implementation uses the "brute force" technique of scanning
888      * over the source list, looking for a match with the target at each
889      * location in turn.
890      *
891      * @param source the list in which to search for the first occurrence
892      *        of <tt>target</tt>.
893      * @param target the list to search for as a subList of <tt>source</tt>.
894      * @return the starting position of the first occurrence of the specified
895      *         target list within the specified source list, or -1 if there
896      *         is no such occurrence.
897      * @since  1.4
898      */
indexOfSubList(List<?> source, List<?> target)899     public static int indexOfSubList(List<?> source, List<?> target) {
900         int sourceSize = source.size();
901         int targetSize = target.size();
902         int maxCandidate = sourceSize - targetSize;
903 
904         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
905             (source instanceof RandomAccess&&target instanceof RandomAccess)) {
906         nextCand:
907             for (int candidate = 0; candidate <= maxCandidate; candidate++) {
908                 for (int i=0, j=candidate; i<targetSize; i++, j++)
909                     if (!eq(target.get(i), source.get(j)))
910                         continue nextCand;  // Element mismatch, try next cand
911                 return candidate;  // All elements of candidate matched target
912             }
913         } else {  // Iterator version of above algorithm
914             ListIterator<?> si = source.listIterator();
915         nextCand:
916             for (int candidate = 0; candidate <= maxCandidate; candidate++) {
917                 ListIterator<?> ti = target.listIterator();
918                 for (int i=0; i<targetSize; i++) {
919                     if (!eq(ti.next(), si.next())) {
920                         // Back up source iterator to next candidate
921                         for (int j=0; j<i; j++)
922                             si.previous();
923                         continue nextCand;
924                     }
925                 }
926                 return candidate;
927             }
928         }
929         return -1;  // No candidate matched the target
930     }
931 
932     /**
933      * Returns the starting position of the last occurrence of the specified
934      * target list within the specified source list, or -1 if there is no such
935      * occurrence.  More formally, returns the highest index <tt>i</tt>
936      * such that {@code source.subList(i, i+target.size()).equals(target)},
937      * or -1 if there is no such index.  (Returns -1 if
938      * {@code target.size() > source.size()})
939      *
940      * <p>This implementation uses the "brute force" technique of iterating
941      * over the source list, looking for a match with the target at each
942      * location in turn.
943      *
944      * @param source the list in which to search for the last occurrence
945      *        of <tt>target</tt>.
946      * @param target the list to search for as a subList of <tt>source</tt>.
947      * @return the starting position of the last occurrence of the specified
948      *         target list within the specified source list, or -1 if there
949      *         is no such occurrence.
950      * @since  1.4
951      */
lastIndexOfSubList(List<?> source, List<?> target)952     public static int lastIndexOfSubList(List<?> source, List<?> target) {
953         int sourceSize = source.size();
954         int targetSize = target.size();
955         int maxCandidate = sourceSize - targetSize;
956 
957         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
958             source instanceof RandomAccess) {   // Index access version
959         nextCand:
960             for (int candidate = maxCandidate; candidate >= 0; candidate--) {
961                 for (int i=0, j=candidate; i<targetSize; i++, j++)
962                     if (!eq(target.get(i), source.get(j)))
963                         continue nextCand;  // Element mismatch, try next cand
964                 return candidate;  // All elements of candidate matched target
965             }
966         } else {  // Iterator version of above algorithm
967             if (maxCandidate < 0)
968                 return -1;
969             ListIterator<?> si = source.listIterator(maxCandidate);
970         nextCand:
971             for (int candidate = maxCandidate; candidate >= 0; candidate--) {
972                 ListIterator<?> ti = target.listIterator();
973                 for (int i=0; i<targetSize; i++) {
974                     if (!eq(ti.next(), si.next())) {
975                         if (candidate != 0) {
976                             // Back up source iterator to next candidate
977                             for (int j=0; j<=i+1; j++)
978                                 si.previous();
979                         }
980                         continue nextCand;
981                     }
982                 }
983                 return candidate;
984             }
985         }
986         return -1;  // No candidate matched the target
987     }
988 
989 
990     // Unmodifiable Wrappers
991 
992     /**
993      * Returns an unmodifiable view of the specified collection.  This method
994      * allows modules to provide users with "read-only" access to internal
995      * collections.  Query operations on the returned collection "read through"
996      * to the specified collection, and attempts to modify the returned
997      * collection, whether direct or via its iterator, result in an
998      * <tt>UnsupportedOperationException</tt>.<p>
999      *
1000      * The returned collection does <i>not</i> pass the hashCode and equals
1001      * operations through to the backing collection, but relies on
1002      * <tt>Object</tt>'s <tt>equals</tt> and <tt>hashCode</tt> methods.  This
1003      * is necessary to preserve the contracts of these operations in the case
1004      * that the backing collection is a set or a list.<p>
1005      *
1006      * The returned collection will be serializable if the specified collection
1007      * is serializable.
1008      *
1009      * @param  <T> the class of the objects in the collection
1010      * @param  c the collection for which an unmodifiable view is to be
1011      *         returned.
1012      * @return an unmodifiable view of the specified collection.
1013      */
unmodifiableCollection(Collection<? extends T> c)1014     public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
1015         return new UnmodifiableCollection<>(c);
1016     }
1017 
1018     /**
1019      * @serial include
1020      */
1021     static class UnmodifiableCollection<E> implements Collection<E>, Serializable {
1022         private static final long serialVersionUID = 1820017752578914078L;
1023 
1024         final Collection<? extends E> c;
1025 
UnmodifiableCollection(Collection<? extends E> c)1026         UnmodifiableCollection(Collection<? extends E> c) {
1027             if (c==null)
1028                 throw new NullPointerException();
1029             this.c = c;
1030         }
1031 
size()1032         public int size()                   {return c.size();}
isEmpty()1033         public boolean isEmpty()            {return c.isEmpty();}
contains(Object o)1034         public boolean contains(Object o)   {return c.contains(o);}
toArray()1035         public Object[] toArray()           {return c.toArray();}
toArray(T[] a)1036         public <T> T[] toArray(T[] a)       {return c.toArray(a);}
toString()1037         public String toString()            {return c.toString();}
1038 
iterator()1039         public Iterator<E> iterator() {
1040             return new Iterator<E>() {
1041                 private final Iterator<? extends E> i = c.iterator();
1042 
1043                 public boolean hasNext() {return i.hasNext();}
1044                 public E next()          {return i.next();}
1045                 public void remove() {
1046                     throw new UnsupportedOperationException();
1047                 }
1048                 @Override
1049                 public void forEachRemaining(Consumer<? super E> action) {
1050                     // Use backing collection version
1051                     i.forEachRemaining(action);
1052                 }
1053             };
1054         }
1055 
add(E e)1056         public boolean add(E e) {
1057             throw new UnsupportedOperationException();
1058         }
remove(Object o)1059         public boolean remove(Object o) {
1060             throw new UnsupportedOperationException();
1061         }
1062 
containsAll(Collection<?> coll)1063         public boolean containsAll(Collection<?> coll) {
1064             return c.containsAll(coll);
1065         }
addAll(Collection<? extends E> coll)1066         public boolean addAll(Collection<? extends E> coll) {
1067             throw new UnsupportedOperationException();
1068         }
removeAll(Collection<?> coll)1069         public boolean removeAll(Collection<?> coll) {
1070             throw new UnsupportedOperationException();
1071         }
retainAll(Collection<?> coll)1072         public boolean retainAll(Collection<?> coll) {
1073             throw new UnsupportedOperationException();
1074         }
clear()1075         public void clear() {
1076             throw new UnsupportedOperationException();
1077         }
1078 
1079         // Override default methods in Collection
1080         @Override
forEach(Consumer<? super E> action)1081         public void forEach(Consumer<? super E> action) {
1082             c.forEach(action);
1083         }
1084         @Override
removeIf(Predicate<? super E> filter)1085         public boolean removeIf(Predicate<? super E> filter) {
1086             throw new UnsupportedOperationException();
1087         }
1088         @SuppressWarnings("unchecked")
1089         @Override
spliterator()1090         public Spliterator<E> spliterator() {
1091             return (Spliterator<E>)c.spliterator();
1092         }
1093         @SuppressWarnings("unchecked")
1094         @Override
stream()1095         public Stream<E> stream() {
1096             return (Stream<E>)c.stream();
1097         }
1098         @SuppressWarnings("unchecked")
1099         @Override
parallelStream()1100         public Stream<E> parallelStream() {
1101             return (Stream<E>)c.parallelStream();
1102         }
1103     }
1104 
1105     /**
1106      * Returns an unmodifiable view of the specified set.  This method allows
1107      * modules to provide users with "read-only" access to internal sets.
1108      * Query operations on the returned set "read through" to the specified
1109      * set, and attempts to modify the returned set, whether direct or via its
1110      * iterator, result in an <tt>UnsupportedOperationException</tt>.<p>
1111      *
1112      * The returned set will be serializable if the specified set
1113      * is serializable.
1114      *
1115      * @param  <T> the class of the objects in the set
1116      * @param  s the set for which an unmodifiable view is to be returned.
1117      * @return an unmodifiable view of the specified set.
1118      */
unmodifiableSet(Set<? extends T> s)1119     public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {
1120         return new UnmodifiableSet<>(s);
1121     }
1122 
1123     /**
1124      * @serial include
1125      */
1126     static class UnmodifiableSet<E> extends UnmodifiableCollection<E>
1127                                  implements Set<E>, Serializable {
1128         private static final long serialVersionUID = -9215047833775013803L;
1129 
UnmodifiableSet(Set<? extends E> s)1130         UnmodifiableSet(Set<? extends E> s)     {super(s);}
equals(Object o)1131         public boolean equals(Object o) {return o == this || c.equals(o);}
hashCode()1132         public int hashCode()           {return c.hashCode();}
1133     }
1134 
1135     /**
1136      * Returns an unmodifiable view of the specified sorted set.  This method
1137      * allows modules to provide users with "read-only" access to internal
1138      * sorted sets.  Query operations on the returned sorted set "read
1139      * through" to the specified sorted set.  Attempts to modify the returned
1140      * sorted set, whether direct, via its iterator, or via its
1141      * <tt>subSet</tt>, <tt>headSet</tt>, or <tt>tailSet</tt> views, result in
1142      * an <tt>UnsupportedOperationException</tt>.<p>
1143      *
1144      * The returned sorted set will be serializable if the specified sorted set
1145      * is serializable.
1146      *
1147      * @param  <T> the class of the objects in the set
1148      * @param s the sorted set for which an unmodifiable view is to be
1149      *        returned.
1150      * @return an unmodifiable view of the specified sorted set.
1151      */
unmodifiableSortedSet(SortedSet<T> s)1152     public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) {
1153         return new UnmodifiableSortedSet<>(s);
1154     }
1155 
1156     /**
1157      * @serial include
1158      */
1159     static class UnmodifiableSortedSet<E>
1160                              extends UnmodifiableSet<E>
1161                              implements SortedSet<E>, Serializable {
1162         private static final long serialVersionUID = -4929149591599911165L;
1163         private final SortedSet<E> ss;
1164 
UnmodifiableSortedSet(SortedSet<E> s)1165         UnmodifiableSortedSet(SortedSet<E> s) {super(s); ss = s;}
1166 
comparator()1167         public Comparator<? super E> comparator() {return ss.comparator();}
1168 
subSet(E fromElement, E toElement)1169         public SortedSet<E> subSet(E fromElement, E toElement) {
1170             return new UnmodifiableSortedSet<>(ss.subSet(fromElement,toElement));
1171         }
headSet(E toElement)1172         public SortedSet<E> headSet(E toElement) {
1173             return new UnmodifiableSortedSet<>(ss.headSet(toElement));
1174         }
tailSet(E fromElement)1175         public SortedSet<E> tailSet(E fromElement) {
1176             return new UnmodifiableSortedSet<>(ss.tailSet(fromElement));
1177         }
1178 
first()1179         public E first()                   {return ss.first();}
last()1180         public E last()                    {return ss.last();}
1181     }
1182 
1183     /**
1184      * Returns an unmodifiable view of the specified navigable set.  This method
1185      * allows modules to provide users with "read-only" access to internal
1186      * navigable sets.  Query operations on the returned navigable set "read
1187      * through" to the specified navigable set.  Attempts to modify the returned
1188      * navigable set, whether direct, via its iterator, or via its
1189      * {@code subSet}, {@code headSet}, or {@code tailSet} views, result in
1190      * an {@code UnsupportedOperationException}.<p>
1191      *
1192      * The returned navigable set will be serializable if the specified
1193      * navigable set is serializable.
1194      *
1195      * @param  <T> the class of the objects in the set
1196      * @param s the navigable set for which an unmodifiable view is to be
1197      *        returned
1198      * @return an unmodifiable view of the specified navigable set
1199      * @since 1.8
1200      */
unmodifiableNavigableSet(NavigableSet<T> s)1201     public static <T> NavigableSet<T> unmodifiableNavigableSet(NavigableSet<T> s) {
1202         return new UnmodifiableNavigableSet<>(s);
1203     }
1204 
1205     /**
1206      * Wraps a navigable set and disables all of the mutative operations.
1207      *
1208      * @param <E> type of elements
1209      * @serial include
1210      */
1211     static class UnmodifiableNavigableSet<E>
1212                              extends UnmodifiableSortedSet<E>
1213                              implements NavigableSet<E>, Serializable {
1214 
1215         private static final long serialVersionUID = -6027448201786391929L;
1216 
1217         /**
1218          * A singleton empty unmodifiable navigable set used for
1219          * {@link #emptyNavigableSet()}.
1220          *
1221          * @param <E> type of elements, if there were any, and bounds
1222          */
1223         private static class EmptyNavigableSet<E> extends UnmodifiableNavigableSet<E>
1224             implements Serializable {
1225             private static final long serialVersionUID = -6291252904449939134L;
1226 
EmptyNavigableSet()1227             public EmptyNavigableSet() {
1228                 super(new TreeSet<E>());
1229             }
1230 
readResolve()1231             private Object readResolve()        { return EMPTY_NAVIGABLE_SET; }
1232         }
1233 
1234         @SuppressWarnings("rawtypes")
1235         private static final NavigableSet<?> EMPTY_NAVIGABLE_SET =
1236                 new EmptyNavigableSet<>();
1237 
1238         /**
1239          * The instance we are protecting.
1240          */
1241         private final NavigableSet<E> ns;
1242 
UnmodifiableNavigableSet(NavigableSet<E> s)1243         UnmodifiableNavigableSet(NavigableSet<E> s)         {super(s); ns = s;}
1244 
lower(E e)1245         public E lower(E e)                             { return ns.lower(e); }
floor(E e)1246         public E floor(E e)                             { return ns.floor(e); }
ceiling(E e)1247         public E ceiling(E e)                         { return ns.ceiling(e); }
higher(E e)1248         public E higher(E e)                           { return ns.higher(e); }
pollFirst()1249         public E pollFirst()     { throw new UnsupportedOperationException(); }
pollLast()1250         public E pollLast()      { throw new UnsupportedOperationException(); }
descendingSet()1251         public NavigableSet<E> descendingSet()
1252                  { return new UnmodifiableNavigableSet<>(ns.descendingSet()); }
descendingIterator()1253         public Iterator<E> descendingIterator()
1254                                          { return descendingSet().iterator(); }
1255 
subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)1256         public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
1257             return new UnmodifiableNavigableSet<>(
1258                 ns.subSet(fromElement, fromInclusive, toElement, toInclusive));
1259         }
1260 
headSet(E toElement, boolean inclusive)1261         public NavigableSet<E> headSet(E toElement, boolean inclusive) {
1262             return new UnmodifiableNavigableSet<>(
1263                 ns.headSet(toElement, inclusive));
1264         }
1265 
tailSet(E fromElement, boolean inclusive)1266         public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
1267             return new UnmodifiableNavigableSet<>(
1268                 ns.tailSet(fromElement, inclusive));
1269         }
1270     }
1271 
1272     /**
1273      * Returns an unmodifiable view of the specified list.  This method allows
1274      * modules to provide users with "read-only" access to internal
1275      * lists.  Query operations on the returned list "read through" to the
1276      * specified list, and attempts to modify the returned list, whether
1277      * direct or via its iterator, result in an
1278      * <tt>UnsupportedOperationException</tt>.<p>
1279      *
1280      * The returned list will be serializable if the specified list
1281      * is serializable. Similarly, the returned list will implement
1282      * {@link RandomAccess} if the specified list does.
1283      *
1284      * @param  <T> the class of the objects in the list
1285      * @param  list the list for which an unmodifiable view is to be returned.
1286      * @return an unmodifiable view of the specified list.
1287      */
unmodifiableList(List<? extends T> list)1288     public static <T> List<T> unmodifiableList(List<? extends T> list) {
1289         return (list instanceof RandomAccess ?
1290                 new UnmodifiableRandomAccessList<>(list) :
1291                 new UnmodifiableList<>(list));
1292     }
1293 
1294     /**
1295      * @serial include
1296      */
1297     static class UnmodifiableList<E> extends UnmodifiableCollection<E>
1298                                   implements List<E> {
1299         private static final long serialVersionUID = -283967356065247728L;
1300 
1301         final List<? extends E> list;
1302 
UnmodifiableList(List<? extends E> list)1303         UnmodifiableList(List<? extends E> list) {
1304             super(list);
1305             this.list = list;
1306         }
1307 
equals(Object o)1308         public boolean equals(Object o) {return o == this || list.equals(o);}
hashCode()1309         public int hashCode()           {return list.hashCode();}
1310 
get(int index)1311         public E get(int index) {return list.get(index);}
set(int index, E element)1312         public E set(int index, E element) {
1313             throw new UnsupportedOperationException();
1314         }
add(int index, E element)1315         public void add(int index, E element) {
1316             throw new UnsupportedOperationException();
1317         }
remove(int index)1318         public E remove(int index) {
1319             throw new UnsupportedOperationException();
1320         }
indexOf(Object o)1321         public int indexOf(Object o)            {return list.indexOf(o);}
lastIndexOf(Object o)1322         public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}
addAll(int index, Collection<? extends E> c)1323         public boolean addAll(int index, Collection<? extends E> c) {
1324             throw new UnsupportedOperationException();
1325         }
1326 
1327         @Override
replaceAll(UnaryOperator<E> operator)1328         public void replaceAll(UnaryOperator<E> operator) {
1329             throw new UnsupportedOperationException();
1330         }
1331         @Override
sort(Comparator<? super E> c)1332         public void sort(Comparator<? super E> c) {
1333             throw new UnsupportedOperationException();
1334         }
1335 
listIterator()1336         public ListIterator<E> listIterator()   {return listIterator(0);}
1337 
listIterator(final int index)1338         public ListIterator<E> listIterator(final int index) {
1339             return new ListIterator<E>() {
1340                 private final ListIterator<? extends E> i
1341                     = list.listIterator(index);
1342 
1343                 public boolean hasNext()     {return i.hasNext();}
1344                 public E next()              {return i.next();}
1345                 public boolean hasPrevious() {return i.hasPrevious();}
1346                 public E previous()          {return i.previous();}
1347                 public int nextIndex()       {return i.nextIndex();}
1348                 public int previousIndex()   {return i.previousIndex();}
1349 
1350                 public void remove() {
1351                     throw new UnsupportedOperationException();
1352                 }
1353                 public void set(E e) {
1354                     throw new UnsupportedOperationException();
1355                 }
1356                 public void add(E e) {
1357                     throw new UnsupportedOperationException();
1358                 }
1359 
1360                 @Override
1361                 public void forEachRemaining(Consumer<? super E> action) {
1362                     i.forEachRemaining(action);
1363                 }
1364             };
1365         }
1366 
subList(int fromIndex, int toIndex)1367         public List<E> subList(int fromIndex, int toIndex) {
1368             return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
1369         }
1370 
1371         /**
1372          * UnmodifiableRandomAccessList instances are serialized as
1373          * UnmodifiableList instances to allow them to be deserialized
1374          * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
1375          * This method inverts the transformation.  As a beneficial
1376          * side-effect, it also grafts the RandomAccess marker onto
1377          * UnmodifiableList instances that were serialized in pre-1.4 JREs.
1378          *
1379          * Note: Unfortunately, UnmodifiableRandomAccessList instances
1380          * serialized in 1.4.1 and deserialized in 1.4 will become
1381          * UnmodifiableList instances, as this method was missing in 1.4.
1382          */
readResolve()1383         private Object readResolve() {
1384             return (list instanceof RandomAccess
1385                     ? new UnmodifiableRandomAccessList<>(list)
1386                     : this);
1387         }
1388     }
1389 
1390     /**
1391      * @serial include
1392      */
1393     static class UnmodifiableRandomAccessList<E> extends UnmodifiableList<E>
1394                                               implements RandomAccess
1395     {
1396         UnmodifiableRandomAccessList(List<? extends E> list) {
1397             super(list);
1398         }
1399 
1400         public List<E> subList(int fromIndex, int toIndex) {
1401             return new UnmodifiableRandomAccessList<>(
1402                 list.subList(fromIndex, toIndex));
1403         }
1404 
1405         private static final long serialVersionUID = -2542308836966382001L;
1406 
1407         /**
1408          * Allows instances to be deserialized in pre-1.4 JREs (which do
1409          * not have UnmodifiableRandomAccessList).  UnmodifiableList has
1410          * a readResolve method that inverts this transformation upon
1411          * deserialization.
1412          */
1413         private Object writeReplace() {
1414             return new UnmodifiableList<>(list);
1415         }
1416     }
1417 
1418     /**
1419      * Returns an unmodifiable view of the specified map.  This method
1420      * allows modules to provide users with "read-only" access to internal
1421      * maps.  Query operations on the returned map "read through"
1422      * to the specified map, and attempts to modify the returned
1423      * map, whether direct or via its collection views, result in an
1424      * <tt>UnsupportedOperationException</tt>.<p>
1425      *
1426      * The returned map will be serializable if the specified map
1427      * is serializable.
1428      *
1429      * @param <K> the class of the map keys
1430      * @param <V> the class of the map values
1431      * @param  m the map for which an unmodifiable view is to be returned.
1432      * @return an unmodifiable view of the specified map.
1433      */
1434     public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {
1435         return new UnmodifiableMap<>(m);
1436     }
1437 
1438     /**
1439      * @serial include
1440      */
1441     private static class UnmodifiableMap<K,V> implements Map<K,V>, Serializable {
1442         private static final long serialVersionUID = -1034234728574286014L;
1443 
1444         private final Map<? extends K, ? extends V> m;
1445 
1446         UnmodifiableMap(Map<? extends K, ? extends V> m) {
1447             if (m==null)
1448                 throw new NullPointerException();
1449             this.m = m;
1450         }
1451 
1452         public int size()                        {return m.size();}
1453         public boolean isEmpty()                 {return m.isEmpty();}
1454         public boolean containsKey(Object key)   {return m.containsKey(key);}
1455         public boolean containsValue(Object val) {return m.containsValue(val);}
1456         public V get(Object key)                 {return m.get(key);}
1457 
1458         public V put(K key, V value) {
1459             throw new UnsupportedOperationException();
1460         }
1461         public V remove(Object key) {
1462             throw new UnsupportedOperationException();
1463         }
1464         public void putAll(Map<? extends K, ? extends V> m) {
1465             throw new UnsupportedOperationException();
1466         }
1467         public void clear() {
1468             throw new UnsupportedOperationException();
1469         }
1470 
1471         private transient Set<K> keySet;
1472         private transient Set<Map.Entry<K,V>> entrySet;
1473         private transient Collection<V> values;
1474 
1475         public Set<K> keySet() {
1476             if (keySet==null)
1477                 keySet = unmodifiableSet(m.keySet());
1478             return keySet;
1479         }
1480 
1481         public Set<Map.Entry<K,V>> entrySet() {
1482             if (entrySet==null)
1483                 entrySet = new UnmodifiableEntrySet<>(m.entrySet());
1484             return entrySet;
1485         }
1486 
1487         public Collection<V> values() {
1488             if (values==null)
1489                 values = unmodifiableCollection(m.values());
1490             return values;
1491         }
1492 
1493         public boolean equals(Object o) {return o == this || m.equals(o);}
1494         public int hashCode()           {return m.hashCode();}
1495         public String toString()        {return m.toString();}
1496 
1497         // Override default methods in Map
1498         @Override
1499         @SuppressWarnings("unchecked")
1500         public V getOrDefault(Object k, V defaultValue) {
1501             // Safe cast as we don't change the value
1502             return ((Map<K, V>)m).getOrDefault(k, defaultValue);
1503         }
1504 
1505         @Override
1506         public void forEach(BiConsumer<? super K, ? super V> action) {
1507             m.forEach(action);
1508         }
1509 
1510         @Override
1511         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
1512             throw new UnsupportedOperationException();
1513         }
1514 
1515         @Override
1516         public V putIfAbsent(K key, V value) {
1517             throw new UnsupportedOperationException();
1518         }
1519 
1520         @Override
1521         public boolean remove(Object key, Object value) {
1522             throw new UnsupportedOperationException();
1523         }
1524 
1525         @Override
1526         public boolean replace(K key, V oldValue, V newValue) {
1527             throw new UnsupportedOperationException();
1528         }
1529 
1530         @Override
1531         public V replace(K key, V value) {
1532             throw new UnsupportedOperationException();
1533         }
1534 
1535         @Override
1536         public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
1537             throw new UnsupportedOperationException();
1538         }
1539 
1540         @Override
1541         public V computeIfPresent(K key,
1542                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1543             throw new UnsupportedOperationException();
1544         }
1545 
1546         @Override
1547         public V compute(K key,
1548                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1549             throw new UnsupportedOperationException();
1550         }
1551 
1552         @Override
1553         public V merge(K key, V value,
1554                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
1555             throw new UnsupportedOperationException();
1556         }
1557 
1558         /**
1559          * We need this class in addition to UnmodifiableSet as
1560          * Map.Entries themselves permit modification of the backing Map
1561          * via their setValue operation.  This class is subtle: there are
1562          * many possible attacks that must be thwarted.
1563          *
1564          * @serial include
1565          */
1566         static class UnmodifiableEntrySet<K,V>
1567             extends UnmodifiableSet<Map.Entry<K,V>> {
1568             private static final long serialVersionUID = 7854390611657943733L;
1569 
1570             @SuppressWarnings({"unchecked", "rawtypes"})
1571             UnmodifiableEntrySet(Set<? extends Map.Entry<? extends K, ? extends V>> s) {
1572                 // Need to cast to raw in order to work around a limitation in the type system
1573                 super((Set)s);
1574             }
1575 
1576             static <K, V> Consumer<Map.Entry<K, V>> entryConsumer(Consumer<? super Entry<K, V>> action) {
1577                 return e -> action.accept(new UnmodifiableEntry<>(e));
1578             }
1579 
1580             public void forEach(Consumer<? super Entry<K, V>> action) {
1581                 Objects.requireNonNull(action);
1582                 c.forEach(entryConsumer(action));
1583             }
1584 
1585             static final class UnmodifiableEntrySetSpliterator<K, V>
1586                     implements Spliterator<Entry<K,V>> {
1587                 final Spliterator<Map.Entry<K, V>> s;
1588 
1589                 UnmodifiableEntrySetSpliterator(Spliterator<Entry<K, V>> s) {
1590                     this.s = s;
1591                 }
1592 
1593                 @Override
1594                 public boolean tryAdvance(Consumer<? super Entry<K, V>> action) {
1595                     Objects.requireNonNull(action);
1596                     return s.tryAdvance(entryConsumer(action));
1597                 }
1598 
1599                 @Override
1600                 public void forEachRemaining(Consumer<? super Entry<K, V>> action) {
1601                     Objects.requireNonNull(action);
1602                     s.forEachRemaining(entryConsumer(action));
1603                 }
1604 
1605                 @Override
1606                 public Spliterator<Entry<K, V>> trySplit() {
1607                     Spliterator<Entry<K, V>> split = s.trySplit();
1608                     return split == null
1609                            ? null
1610                            : new UnmodifiableEntrySetSpliterator<>(split);
1611                 }
1612 
1613                 @Override
1614                 public long estimateSize() {
1615                     return s.estimateSize();
1616                 }
1617 
1618                 @Override
1619                 public long getExactSizeIfKnown() {
1620                     return s.getExactSizeIfKnown();
1621                 }
1622 
1623                 @Override
1624                 public int characteristics() {
1625                     return s.characteristics();
1626                 }
1627 
1628                 @Override
1629                 public boolean hasCharacteristics(int characteristics) {
1630                     return s.hasCharacteristics(characteristics);
1631                 }
1632 
1633                 @Override
1634                 public Comparator<? super Entry<K, V>> getComparator() {
1635                     return s.getComparator();
1636                 }
1637             }
1638 
1639             @SuppressWarnings("unchecked")
1640             public Spliterator<Entry<K,V>> spliterator() {
1641                 return new UnmodifiableEntrySetSpliterator<>(
1642                         (Spliterator<Map.Entry<K, V>>) c.spliterator());
1643             }
1644 
1645             @Override
1646             public Stream<Entry<K,V>> stream() {
1647                 return StreamSupport.stream(spliterator(), false);
1648             }
1649 
1650             @Override
1651             public Stream<Entry<K,V>> parallelStream() {
1652                 return StreamSupport.stream(spliterator(), true);
1653             }
1654 
1655             public Iterator<Map.Entry<K,V>> iterator() {
1656                 return new Iterator<Map.Entry<K,V>>() {
1657                     private final Iterator<? extends Map.Entry<? extends K, ? extends V>> i = c.iterator();
1658 
1659                     public boolean hasNext() {
1660                         return i.hasNext();
1661                     }
1662                     public Map.Entry<K,V> next() {
1663                         return new UnmodifiableEntry<>(i.next());
1664                     }
1665                     public void remove() {
1666                         throw new UnsupportedOperationException();
1667                     }
1668                 };
1669             }
1670 
1671             @SuppressWarnings("unchecked")
1672             public Object[] toArray() {
1673                 Object[] a = c.toArray();
1674                 for (int i=0; i<a.length; i++)
1675                     a[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>)a[i]);
1676                 return a;
1677             }
1678 
1679             @SuppressWarnings("unchecked")
1680             public <T> T[] toArray(T[] a) {
1681                 // We don't pass a to c.toArray, to avoid window of
1682                 // vulnerability wherein an unscrupulous multithreaded client
1683                 // could get his hands on raw (unwrapped) Entries from c.
1684                 Object[] arr = c.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
1685 
1686                 for (int i=0; i<arr.length; i++)
1687                     arr[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>)arr[i]);
1688 
1689                 if (arr.length > a.length)
1690                     return (T[])arr;
1691 
1692                 System.arraycopy(arr, 0, a, 0, arr.length);
1693                 if (a.length > arr.length)
1694                     a[arr.length] = null;
1695                 return a;
1696             }
1697 
1698             /**
1699              * This method is overridden to protect the backing set against
1700              * an object with a nefarious equals function that senses
1701              * that the equality-candidate is Map.Entry and calls its
1702              * setValue method.
1703              */
1704             public boolean contains(Object o) {
1705                 if (!(o instanceof Map.Entry))
1706                     return false;
1707                 return c.contains(
1708                     new UnmodifiableEntry<>((Map.Entry<?,?>) o));
1709             }
1710 
1711             /**
1712              * The next two methods are overridden to protect against
1713              * an unscrupulous List whose contains(Object o) method senses
1714              * when o is a Map.Entry, and calls o.setValue.
1715              */
1716             public boolean containsAll(Collection<?> coll) {
1717                 for (Object e : coll) {
1718                     if (!contains(e)) // Invokes safe contains() above
1719                         return false;
1720                 }
1721                 return true;
1722             }
1723             public boolean equals(Object o) {
1724                 if (o == this)
1725                     return true;
1726 
1727                 if (!(o instanceof Set))
1728                     return false;
1729                 Set<?> s = (Set<?>) o;
1730                 if (s.size() != c.size())
1731                     return false;
1732                 return containsAll(s); // Invokes safe containsAll() above
1733             }
1734 
1735             /**
1736              * This "wrapper class" serves two purposes: it prevents
1737              * the client from modifying the backing Map, by short-circuiting
1738              * the setValue method, and it protects the backing Map against
1739              * an ill-behaved Map.Entry that attempts to modify another
1740              * Map Entry when asked to perform an equality check.
1741              */
1742             private static class UnmodifiableEntry<K,V> implements Map.Entry<K,V> {
1743                 private Map.Entry<? extends K, ? extends V> e;
1744 
1745                 UnmodifiableEntry(Map.Entry<? extends K, ? extends V> e)
1746                         {this.e = Objects.requireNonNull(e);}
1747 
1748                 public K getKey()        {return e.getKey();}
1749                 public V getValue()      {return e.getValue();}
1750                 public V setValue(V value) {
1751                     throw new UnsupportedOperationException();
1752                 }
1753                 public int hashCode()    {return e.hashCode();}
1754                 public boolean equals(Object o) {
1755                     if (this == o)
1756                         return true;
1757                     if (!(o instanceof Map.Entry))
1758                         return false;
1759                     Map.Entry<?,?> t = (Map.Entry<?,?>)o;
1760                     return eq(e.getKey(),   t.getKey()) &&
1761                            eq(e.getValue(), t.getValue());
1762                 }
1763                 public String toString() {return e.toString();}
1764             }
1765         }
1766     }
1767 
1768     /**
1769      * Returns an unmodifiable view of the specified sorted map.  This method
1770      * allows modules to provide users with "read-only" access to internal
1771      * sorted maps.  Query operations on the returned sorted map "read through"
1772      * to the specified sorted map.  Attempts to modify the returned
1773      * sorted map, whether direct, via its collection views, or via its
1774      * <tt>subMap</tt>, <tt>headMap</tt>, or <tt>tailMap</tt> views, result in
1775      * an <tt>UnsupportedOperationException</tt>.<p>
1776      *
1777      * The returned sorted map will be serializable if the specified sorted map
1778      * is serializable.
1779      *
1780      * @param <K> the class of the map keys
1781      * @param <V> the class of the map values
1782      * @param m the sorted map for which an unmodifiable view is to be
1783      *        returned.
1784      * @return an unmodifiable view of the specified sorted map.
1785      */
1786     public static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {
1787         return new UnmodifiableSortedMap<>(m);
1788     }
1789 
1790     /**
1791      * @serial include
1792      */
1793     static class UnmodifiableSortedMap<K,V>
1794           extends UnmodifiableMap<K,V>
1795           implements SortedMap<K,V>, Serializable {
1796         private static final long serialVersionUID = -8806743815996713206L;
1797 
1798         private final SortedMap<K, ? extends V> sm;
1799 
1800         UnmodifiableSortedMap(SortedMap<K, ? extends V> m) {super(m); sm = m; }
1801         public Comparator<? super K> comparator()   { return sm.comparator(); }
1802         public SortedMap<K,V> subMap(K fromKey, K toKey)
1803              { return new UnmodifiableSortedMap<>(sm.subMap(fromKey, toKey)); }
1804         public SortedMap<K,V> headMap(K toKey)
1805                      { return new UnmodifiableSortedMap<>(sm.headMap(toKey)); }
1806         public SortedMap<K,V> tailMap(K fromKey)
1807                    { return new UnmodifiableSortedMap<>(sm.tailMap(fromKey)); }
1808         public K firstKey()                           { return sm.firstKey(); }
1809         public K lastKey()                             { return sm.lastKey(); }
1810     }
1811 
1812     /**
1813      * Returns an unmodifiable view of the specified navigable map.  This method
1814      * allows modules to provide users with "read-only" access to internal
1815      * navigable maps.  Query operations on the returned navigable map "read
1816      * through" to the specified navigable map.  Attempts to modify the returned
1817      * navigable map, whether direct, via its collection views, or via its
1818      * {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
1819      * an {@code UnsupportedOperationException}.<p>
1820      *
1821      * The returned navigable map will be serializable if the specified
1822      * navigable map is serializable.
1823      *
1824      * @param <K> the class of the map keys
1825      * @param <V> the class of the map values
1826      * @param m the navigable map for which an unmodifiable view is to be
1827      *        returned
1828      * @return an unmodifiable view of the specified navigable map
1829      * @since 1.8
1830      */
1831     public static <K,V> NavigableMap<K,V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) {
1832         return new UnmodifiableNavigableMap<>(m);
1833     }
1834 
1835     /**
1836      * @serial include
1837      */
1838     static class UnmodifiableNavigableMap<K,V>
1839           extends UnmodifiableSortedMap<K,V>
1840           implements NavigableMap<K,V>, Serializable {
1841         private static final long serialVersionUID = -4858195264774772197L;
1842 
1843         /**
1844          * A class for the {@link EMPTY_NAVIGABLE_MAP} which needs readResolve
1845          * to preserve singleton property.
1846          *
1847          * @param <K> type of keys, if there were any, and of bounds
1848          * @param <V> type of values, if there were any
1849          */
1850         private static class EmptyNavigableMap<K,V> extends UnmodifiableNavigableMap<K,V>
1851             implements Serializable {
1852 
1853             private static final long serialVersionUID = -2239321462712562324L;
1854 
1855             EmptyNavigableMap()                       { super(new TreeMap<K,V>()); }
1856 
1857             @Override
1858             public NavigableSet<K> navigableKeySet()
1859                                                 { return emptyNavigableSet(); }
1860 
1861             private Object readResolve()        { return EMPTY_NAVIGABLE_MAP; }
1862         }
1863 
1864         /**
1865          * Singleton for {@link emptyNavigableMap()} which is also immutable.
1866          */
1867         private static final EmptyNavigableMap<?,?> EMPTY_NAVIGABLE_MAP =
1868             new EmptyNavigableMap<>();
1869 
1870         /**
1871          * The instance we wrap and protect.
1872          */
1873         private final NavigableMap<K, ? extends V> nm;
1874 
1875         UnmodifiableNavigableMap(NavigableMap<K, ? extends V> m)
1876                                                             {super(m); nm = m;}
1877 
1878         public K lowerKey(K key)                   { return nm.lowerKey(key); }
1879         public K floorKey(K key)                   { return nm.floorKey(key); }
1880         public K ceilingKey(K key)               { return nm.ceilingKey(key); }
1881         public K higherKey(K key)                 { return nm.higherKey(key); }
1882 
1883         @SuppressWarnings("unchecked")
1884         public Entry<K, V> lowerEntry(K key) {
1885             Entry<K,V> lower = (Entry<K, V>) nm.lowerEntry(key);
1886             return (null != lower)
1887                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(lower)
1888                 : null;
1889         }
1890 
1891         @SuppressWarnings("unchecked")
1892         public Entry<K, V> floorEntry(K key) {
1893             Entry<K,V> floor = (Entry<K, V>) nm.floorEntry(key);
1894             return (null != floor)
1895                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(floor)
1896                 : null;
1897         }
1898 
1899         @SuppressWarnings("unchecked")
1900         public Entry<K, V> ceilingEntry(K key) {
1901             Entry<K,V> ceiling = (Entry<K, V>) nm.ceilingEntry(key);
1902             return (null != ceiling)
1903                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(ceiling)
1904                 : null;
1905         }
1906 
1907 
1908         @SuppressWarnings("unchecked")
1909         public Entry<K, V> higherEntry(K key) {
1910             Entry<K,V> higher = (Entry<K, V>) nm.higherEntry(key);
1911             return (null != higher)
1912                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(higher)
1913                 : null;
1914         }
1915 
1916         @SuppressWarnings("unchecked")
1917         public Entry<K, V> firstEntry() {
1918             Entry<K,V> first = (Entry<K, V>) nm.firstEntry();
1919             return (null != first)
1920                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(first)
1921                 : null;
1922         }
1923 
1924         @SuppressWarnings("unchecked")
1925         public Entry<K, V> lastEntry() {
1926             Entry<K,V> last = (Entry<K, V>) nm.lastEntry();
1927             return (null != last)
1928                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(last)
1929                 : null;
1930         }
1931 
1932         public Entry<K, V> pollFirstEntry()
1933                                  { throw new UnsupportedOperationException(); }
1934         public Entry<K, V> pollLastEntry()
1935                                  { throw new UnsupportedOperationException(); }
1936         public NavigableMap<K, V> descendingMap()
1937                        { return unmodifiableNavigableMap(nm.descendingMap()); }
1938         public NavigableSet<K> navigableKeySet()
1939                      { return unmodifiableNavigableSet(nm.navigableKeySet()); }
1940         public NavigableSet<K> descendingKeySet()
1941                     { return unmodifiableNavigableSet(nm.descendingKeySet()); }
1942 
1943         public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
1944             return unmodifiableNavigableMap(
1945                 nm.subMap(fromKey, fromInclusive, toKey, toInclusive));
1946         }
1947 
1948         public NavigableMap<K, V> headMap(K toKey, boolean inclusive)
1949              { return unmodifiableNavigableMap(nm.headMap(toKey, inclusive)); }
1950         public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive)
1951            { return unmodifiableNavigableMap(nm.tailMap(fromKey, inclusive)); }
1952     }
1953 
1954     // Synch Wrappers
1955 
1956     /**
1957      * Returns a synchronized (thread-safe) collection backed by the specified
1958      * collection.  In order to guarantee serial access, it is critical that
1959      * <strong>all</strong> access to the backing collection is accomplished
1960      * through the returned collection.<p>
1961      *
1962      * It is imperative that the user manually synchronize on the returned
1963      * collection when traversing it via {@link Iterator}, {@link Spliterator}
1964      * or {@link Stream}:
1965      * <pre>
1966      *  Collection c = Collections.synchronizedCollection(myCollection);
1967      *     ...
1968      *  synchronized (c) {
1969      *      Iterator i = c.iterator(); // Must be in the synchronized block
1970      *      while (i.hasNext())
1971      *         foo(i.next());
1972      *  }
1973      * </pre>
1974      * Failure to follow this advice may result in non-deterministic behavior.
1975      *
1976      * <p>The returned collection does <i>not</i> pass the {@code hashCode}
1977      * and {@code equals} operations through to the backing collection, but
1978      * relies on {@code Object}'s equals and hashCode methods.  This is
1979      * necessary to preserve the contracts of these operations in the case
1980      * that the backing collection is a set or a list.<p>
1981      *
1982      * The returned collection will be serializable if the specified collection
1983      * is serializable.
1984      *
1985      * @param  <T> the class of the objects in the collection
1986      * @param  c the collection to be "wrapped" in a synchronized collection.
1987      * @return a synchronized view of the specified collection.
1988      */
1989     public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
1990         return new SynchronizedCollection<>(c);
1991     }
1992 
1993     static <T> Collection<T> synchronizedCollection(Collection<T> c, Object mutex) {
1994         return new SynchronizedCollection<>(c, mutex);
1995     }
1996 
1997     /**
1998      * @serial include
1999      */
2000     static class SynchronizedCollection<E> implements Collection<E>, Serializable {
2001         private static final long serialVersionUID = 3053995032091335093L;
2002 
2003         final Collection<E> c;  // Backing Collection
2004         final Object mutex;     // Object on which to synchronize
2005 
2006         SynchronizedCollection(Collection<E> c) {
2007             this.c = Objects.requireNonNull(c);
2008             mutex = this;
2009         }
2010 
2011         SynchronizedCollection(Collection<E> c, Object mutex) {
2012             this.c = Objects.requireNonNull(c);
2013             this.mutex = Objects.requireNonNull(mutex);
2014         }
2015 
2016         public int size() {
2017             synchronized (mutex) {return c.size();}
2018         }
2019         public boolean isEmpty() {
2020             synchronized (mutex) {return c.isEmpty();}
2021         }
2022         public boolean contains(Object o) {
2023             synchronized (mutex) {return c.contains(o);}
2024         }
2025         public Object[] toArray() {
2026             synchronized (mutex) {return c.toArray();}
2027         }
2028         public <T> T[] toArray(T[] a) {
2029             synchronized (mutex) {return c.toArray(a);}
2030         }
2031 
2032         public Iterator<E> iterator() {
2033             return c.iterator(); // Must be manually synched by user!
2034         }
2035 
2036         public boolean add(E e) {
2037             synchronized (mutex) {return c.add(e);}
2038         }
2039         public boolean remove(Object o) {
2040             synchronized (mutex) {return c.remove(o);}
2041         }
2042 
2043         public boolean containsAll(Collection<?> coll) {
2044             synchronized (mutex) {return c.containsAll(coll);}
2045         }
2046         public boolean addAll(Collection<? extends E> coll) {
2047             synchronized (mutex) {return c.addAll(coll);}
2048         }
2049         public boolean removeAll(Collection<?> coll) {
2050             synchronized (mutex) {return c.removeAll(coll);}
2051         }
2052         public boolean retainAll(Collection<?> coll) {
2053             synchronized (mutex) {return c.retainAll(coll);}
2054         }
2055         public void clear() {
2056             synchronized (mutex) {c.clear();}
2057         }
2058         public String toString() {
2059             synchronized (mutex) {return c.toString();}
2060         }
2061         // Override default methods in Collection
2062         @Override
2063         public void forEach(Consumer<? super E> consumer) {
2064             synchronized (mutex) {c.forEach(consumer);}
2065         }
2066         @Override
2067         public boolean removeIf(Predicate<? super E> filter) {
2068             synchronized (mutex) {return c.removeIf(filter);}
2069         }
2070         @Override
2071         public Spliterator<E> spliterator() {
2072             return c.spliterator(); // Must be manually synched by user!
2073         }
2074         @Override
2075         public Stream<E> stream() {
2076             return c.stream(); // Must be manually synched by user!
2077         }
2078         @Override
2079         public Stream<E> parallelStream() {
2080             return c.parallelStream(); // Must be manually synched by user!
2081         }
2082         private void writeObject(ObjectOutputStream s) throws IOException {
2083             synchronized (mutex) {s.defaultWriteObject();}
2084         }
2085     }
2086 
2087     /**
2088      * Returns a synchronized (thread-safe) set backed by the specified
2089      * set.  In order to guarantee serial access, it is critical that
2090      * <strong>all</strong> access to the backing set is accomplished
2091      * through the returned set.<p>
2092      *
2093      * It is imperative that the user manually synchronize on the returned
2094      * set when iterating over it:
2095      * <pre>
2096      *  Set s = Collections.synchronizedSet(new HashSet());
2097      *      ...
2098      *  synchronized (s) {
2099      *      Iterator i = s.iterator(); // Must be in the synchronized block
2100      *      while (i.hasNext())
2101      *          foo(i.next());
2102      *  }
2103      * </pre>
2104      * Failure to follow this advice may result in non-deterministic behavior.
2105      *
2106      * <p>The returned set will be serializable if the specified set is
2107      * serializable.
2108      *
2109      * @param  <T> the class of the objects in the set
2110      * @param  s the set to be "wrapped" in a synchronized set.
2111      * @return a synchronized view of the specified set.
2112      */
2113     public static <T> Set<T> synchronizedSet(Set<T> s) {
2114         return new SynchronizedSet<>(s);
2115     }
2116 
2117     static <T> Set<T> synchronizedSet(Set<T> s, Object mutex) {
2118         return new SynchronizedSet<>(s, mutex);
2119     }
2120 
2121     /**
2122      * @serial include
2123      */
2124     static class SynchronizedSet<E>
2125           extends SynchronizedCollection<E>
2126           implements Set<E> {
2127         private static final long serialVersionUID = 487447009682186044L;
2128 
2129         SynchronizedSet(Set<E> s) {
2130             super(s);
2131         }
2132         SynchronizedSet(Set<E> s, Object mutex) {
2133             super(s, mutex);
2134         }
2135 
2136         public boolean equals(Object o) {
2137             if (this == o)
2138                 return true;
2139             synchronized (mutex) {return c.equals(o);}
2140         }
2141         public int hashCode() {
2142             synchronized (mutex) {return c.hashCode();}
2143         }
2144     }
2145 
2146     /**
2147      * Returns a synchronized (thread-safe) sorted set backed by the specified
2148      * sorted set.  In order to guarantee serial access, it is critical that
2149      * <strong>all</strong> access to the backing sorted set is accomplished
2150      * through the returned sorted set (or its views).<p>
2151      *
2152      * It is imperative that the user manually synchronize on the returned
2153      * sorted set when iterating over it or any of its <tt>subSet</tt>,
2154      * <tt>headSet</tt>, or <tt>tailSet</tt> views.
2155      * <pre>
2156      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
2157      *      ...
2158      *  synchronized (s) {
2159      *      Iterator i = s.iterator(); // Must be in the synchronized block
2160      *      while (i.hasNext())
2161      *          foo(i.next());
2162      *  }
2163      * </pre>
2164      * or:
2165      * <pre>
2166      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
2167      *  SortedSet s2 = s.headSet(foo);
2168      *      ...
2169      *  synchronized (s) {  // Note: s, not s2!!!
2170      *      Iterator i = s2.iterator(); // Must be in the synchronized block
2171      *      while (i.hasNext())
2172      *          foo(i.next());
2173      *  }
2174      * </pre>
2175      * Failure to follow this advice may result in non-deterministic behavior.
2176      *
2177      * <p>The returned sorted set will be serializable if the specified
2178      * sorted set is serializable.
2179      *
2180      * @param  <T> the class of the objects in the set
2181      * @param  s the sorted set to be "wrapped" in a synchronized sorted set.
2182      * @return a synchronized view of the specified sorted set.
2183      */
2184     public static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) {
2185         return new SynchronizedSortedSet<>(s);
2186     }
2187 
2188     /**
2189      * @serial include
2190      */
2191     static class SynchronizedSortedSet<E>
2192         extends SynchronizedSet<E>
2193         implements SortedSet<E>
2194     {
2195         private static final long serialVersionUID = 8695801310862127406L;
2196 
2197         private final SortedSet<E> ss;
2198 
2199         SynchronizedSortedSet(SortedSet<E> s) {
2200             super(s);
2201             ss = s;
2202         }
2203         SynchronizedSortedSet(SortedSet<E> s, Object mutex) {
2204             super(s, mutex);
2205             ss = s;
2206         }
2207 
2208         public Comparator<? super E> comparator() {
2209             synchronized (mutex) {return ss.comparator();}
2210         }
2211 
2212         public SortedSet<E> subSet(E fromElement, E toElement) {
2213             synchronized (mutex) {
2214                 return new SynchronizedSortedSet<>(
2215                     ss.subSet(fromElement, toElement), mutex);
2216             }
2217         }
2218         public SortedSet<E> headSet(E toElement) {
2219             synchronized (mutex) {
2220                 return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);
2221             }
2222         }
2223         public SortedSet<E> tailSet(E fromElement) {
2224             synchronized (mutex) {
2225                return new SynchronizedSortedSet<>(ss.tailSet(fromElement),mutex);
2226             }
2227         }
2228 
2229         public E first() {
2230             synchronized (mutex) {return ss.first();}
2231         }
2232         public E last() {
2233             synchronized (mutex) {return ss.last();}
2234         }
2235     }
2236 
2237     /**
2238      * Returns a synchronized (thread-safe) navigable set backed by the
2239      * specified navigable set.  In order to guarantee serial access, it is
2240      * critical that <strong>all</strong> access to the backing navigable set is
2241      * accomplished through the returned navigable set (or its views).<p>
2242      *
2243      * It is imperative that the user manually synchronize on the returned
2244      * navigable set when iterating over it or any of its {@code subSet},
2245      * {@code headSet}, or {@code tailSet} views.
2246      * <pre>
2247      *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
2248      *      ...
2249      *  synchronized (s) {
2250      *      Iterator i = s.iterator(); // Must be in the synchronized block
2251      *      while (i.hasNext())
2252      *          foo(i.next());
2253      *  }
2254      * </pre>
2255      * or:
2256      * <pre>
2257      *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
2258      *  NavigableSet s2 = s.headSet(foo, true);
2259      *      ...
2260      *  synchronized (s) {  // Note: s, not s2!!!
2261      *      Iterator i = s2.iterator(); // Must be in the synchronized block
2262      *      while (i.hasNext())
2263      *          foo(i.next());
2264      *  }
2265      * </pre>
2266      * Failure to follow this advice may result in non-deterministic behavior.
2267      *
2268      * <p>The returned navigable set will be serializable if the specified
2269      * navigable set is serializable.
2270      *
2271      * @param  <T> the class of the objects in the set
2272      * @param  s the navigable set to be "wrapped" in a synchronized navigable
2273      * set
2274      * @return a synchronized view of the specified navigable set
2275      * @since 1.8
2276      */
2277     public static <T> NavigableSet<T> synchronizedNavigableSet(NavigableSet<T> s) {
2278         return new SynchronizedNavigableSet<>(s);
2279     }
2280 
2281     /**
2282      * @serial include
2283      */
2284     static class SynchronizedNavigableSet<E>
2285         extends SynchronizedSortedSet<E>
2286         implements NavigableSet<E>
2287     {
2288         private static final long serialVersionUID = -5505529816273629798L;
2289 
2290         private final NavigableSet<E> ns;
2291 
2292         SynchronizedNavigableSet(NavigableSet<E> s) {
2293             super(s);
2294             ns = s;
2295         }
2296 
2297         SynchronizedNavigableSet(NavigableSet<E> s, Object mutex) {
2298             super(s, mutex);
2299             ns = s;
2300         }
2301         public E lower(E e)      { synchronized (mutex) {return ns.lower(e);} }
2302         public E floor(E e)      { synchronized (mutex) {return ns.floor(e);} }
2303         public E ceiling(E e)  { synchronized (mutex) {return ns.ceiling(e);} }
2304         public E higher(E e)    { synchronized (mutex) {return ns.higher(e);} }
2305         public E pollFirst()  { synchronized (mutex) {return ns.pollFirst();} }
2306         public E pollLast()    { synchronized (mutex) {return ns.pollLast();} }
2307 
2308         public NavigableSet<E> descendingSet() {
2309             synchronized (mutex) {
2310                 return new SynchronizedNavigableSet<>(ns.descendingSet(), mutex);
2311             }
2312         }
2313 
2314         public Iterator<E> descendingIterator()
2315                  { synchronized (mutex) { return descendingSet().iterator(); } }
2316 
2317         public NavigableSet<E> subSet(E fromElement, E toElement) {
2318             synchronized (mutex) {
2319                 return new SynchronizedNavigableSet<>(ns.subSet(fromElement, true, toElement, false), mutex);
2320             }
2321         }
2322         public NavigableSet<E> headSet(E toElement) {
2323             synchronized (mutex) {
2324                 return new SynchronizedNavigableSet<>(ns.headSet(toElement, false), mutex);
2325             }
2326         }
2327         public NavigableSet<E> tailSet(E fromElement) {
2328             synchronized (mutex) {
2329                 return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, true), mutex);
2330             }
2331         }
2332 
2333         public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
2334             synchronized (mutex) {
2335                 return new SynchronizedNavigableSet<>(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), mutex);
2336             }
2337         }
2338 
2339         public NavigableSet<E> headSet(E toElement, boolean inclusive) {
2340             synchronized (mutex) {
2341                 return new SynchronizedNavigableSet<>(ns.headSet(toElement, inclusive), mutex);
2342             }
2343         }
2344 
2345         public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
2346             synchronized (mutex) {
2347                 return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, inclusive), mutex);
2348             }
2349         }
2350     }
2351 
2352     /**
2353      * Returns a synchronized (thread-safe) list backed by the specified
2354      * list.  In order to guarantee serial access, it is critical that
2355      * <strong>all</strong> access to the backing list is accomplished
2356      * through the returned list.<p>
2357      *
2358      * It is imperative that the user manually synchronize on the returned
2359      * list when iterating over it:
2360      * <pre>
2361      *  List list = Collections.synchronizedList(new ArrayList());
2362      *      ...
2363      *  synchronized (list) {
2364      *      Iterator i = list.iterator(); // Must be in synchronized block
2365      *      while (i.hasNext())
2366      *          foo(i.next());
2367      *  }
2368      * </pre>
2369      * Failure to follow this advice may result in non-deterministic behavior.
2370      *
2371      * <p>The returned list will be serializable if the specified list is
2372      * serializable.
2373      *
2374      * @param  <T> the class of the objects in the list
2375      * @param  list the list to be "wrapped" in a synchronized list.
2376      * @return a synchronized view of the specified list.
2377      */
2378     public static <T> List<T> synchronizedList(List<T> list) {
2379         return (list instanceof RandomAccess ?
2380                 new SynchronizedRandomAccessList<>(list) :
2381                 new SynchronizedList<>(list));
2382     }
2383 
2384     static <T> List<T> synchronizedList(List<T> list, Object mutex) {
2385         return (list instanceof RandomAccess ?
2386                 new SynchronizedRandomAccessList<>(list, mutex) :
2387                 new SynchronizedList<>(list, mutex));
2388     }
2389 
2390     /**
2391      * @serial include
2392      */
2393     static class SynchronizedList<E>
2394         extends SynchronizedCollection<E>
2395         implements List<E> {
2396         private static final long serialVersionUID = -7754090372962971524L;
2397 
2398         final List<E> list;
2399 
2400         SynchronizedList(List<E> list) {
2401             super(list);
2402             this.list = list;
2403         }
2404         SynchronizedList(List<E> list, Object mutex) {
2405             super(list, mutex);
2406             this.list = list;
2407         }
2408 
2409         public boolean equals(Object o) {
2410             if (this == o)
2411                 return true;
2412             synchronized (mutex) {return list.equals(o);}
2413         }
2414         public int hashCode() {
2415             synchronized (mutex) {return list.hashCode();}
2416         }
2417 
2418         public E get(int index) {
2419             synchronized (mutex) {return list.get(index);}
2420         }
2421         public E set(int index, E element) {
2422             synchronized (mutex) {return list.set(index, element);}
2423         }
2424         public void add(int index, E element) {
2425             synchronized (mutex) {list.add(index, element);}
2426         }
2427         public E remove(int index) {
2428             synchronized (mutex) {return list.remove(index);}
2429         }
2430 
2431         public int indexOf(Object o) {
2432             synchronized (mutex) {return list.indexOf(o);}
2433         }
2434         public int lastIndexOf(Object o) {
2435             synchronized (mutex) {return list.lastIndexOf(o);}
2436         }
2437 
2438         public boolean addAll(int index, Collection<? extends E> c) {
2439             synchronized (mutex) {return list.addAll(index, c);}
2440         }
2441 
2442         public ListIterator<E> listIterator() {
2443             return list.listIterator(); // Must be manually synched by user
2444         }
2445 
2446         public ListIterator<E> listIterator(int index) {
2447             return list.listIterator(index); // Must be manually synched by user
2448         }
2449 
2450         public List<E> subList(int fromIndex, int toIndex) {
2451             synchronized (mutex) {
2452                 return new SynchronizedList<>(list.subList(fromIndex, toIndex),
2453                                             mutex);
2454             }
2455         }
2456 
2457         @Override
2458         public void replaceAll(UnaryOperator<E> operator) {
2459             synchronized (mutex) {list.replaceAll(operator);}
2460         }
2461         @Override
2462         public void sort(Comparator<? super E> c) {
2463             synchronized (mutex) {list.sort(c);}
2464         }
2465 
2466         /**
2467          * SynchronizedRandomAccessList instances are serialized as
2468          * SynchronizedList instances to allow them to be deserialized
2469          * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
2470          * This method inverts the transformation.  As a beneficial
2471          * side-effect, it also grafts the RandomAccess marker onto
2472          * SynchronizedList instances that were serialized in pre-1.4 JREs.
2473          *
2474          * Note: Unfortunately, SynchronizedRandomAccessList instances
2475          * serialized in 1.4.1 and deserialized in 1.4 will become
2476          * SynchronizedList instances, as this method was missing in 1.4.
2477          */
2478         private Object readResolve() {
2479             return (list instanceof RandomAccess
2480                     ? new SynchronizedRandomAccessList<>(list)
2481                     : this);
2482         }
2483     }
2484 
2485     /**
2486      * @serial include
2487      */
2488     static class SynchronizedRandomAccessList<E>
2489         extends SynchronizedList<E>
2490         implements RandomAccess {
2491 
2492         SynchronizedRandomAccessList(List<E> list) {
2493             super(list);
2494         }
2495 
2496         SynchronizedRandomAccessList(List<E> list, Object mutex) {
2497             super(list, mutex);
2498         }
2499 
2500         public List<E> subList(int fromIndex, int toIndex) {
2501             synchronized (mutex) {
2502                 return new SynchronizedRandomAccessList<>(
2503                     list.subList(fromIndex, toIndex), mutex);
2504             }
2505         }
2506 
2507         private static final long serialVersionUID = 1530674583602358482L;
2508 
2509         /**
2510          * Allows instances to be deserialized in pre-1.4 JREs (which do
2511          * not have SynchronizedRandomAccessList).  SynchronizedList has
2512          * a readResolve method that inverts this transformation upon
2513          * deserialization.
2514          */
2515         private Object writeReplace() {
2516             return new SynchronizedList<>(list);
2517         }
2518     }
2519 
2520     /**
2521      * Returns a synchronized (thread-safe) map backed by the specified
2522      * map.  In order to guarantee serial access, it is critical that
2523      * <strong>all</strong> access to the backing map is accomplished
2524      * through the returned map.<p>
2525      *
2526      * It is imperative that the user manually synchronize on the returned
2527      * map when iterating over any of its collection views:
2528      * <pre>
2529      *  Map m = Collections.synchronizedMap(new HashMap());
2530      *      ...
2531      *  Set s = m.keySet();  // Needn't be in synchronized block
2532      *      ...
2533      *  synchronized (m) {  // Synchronizing on m, not s!
2534      *      Iterator i = s.iterator(); // Must be in synchronized block
2535      *      while (i.hasNext())
2536      *          foo(i.next());
2537      *  }
2538      * </pre>
2539      * Failure to follow this advice may result in non-deterministic behavior.
2540      *
2541      * <p>The returned map will be serializable if the specified map is
2542      * serializable.
2543      *
2544      * @param <K> the class of the map keys
2545      * @param <V> the class of the map values
2546      * @param  m the map to be "wrapped" in a synchronized map.
2547      * @return a synchronized view of the specified map.
2548      */
2549     public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
2550         return new SynchronizedMap<>(m);
2551     }
2552 
2553     /**
2554      * @serial include
2555      */
2556     private static class SynchronizedMap<K,V>
2557         implements Map<K,V>, Serializable {
2558         private static final long serialVersionUID = 1978198479659022715L;
2559 
2560         private final Map<K,V> m;     // Backing Map
2561         final Object      mutex;        // Object on which to synchronize
2562 
2563         SynchronizedMap(Map<K,V> m) {
2564             this.m = Objects.requireNonNull(m);
2565             mutex = this;
2566         }
2567 
2568         SynchronizedMap(Map<K,V> m, Object mutex) {
2569             this.m = m;
2570             this.mutex = mutex;
2571         }
2572 
2573         public int size() {
2574             synchronized (mutex) {return m.size();}
2575         }
2576         public boolean isEmpty() {
2577             synchronized (mutex) {return m.isEmpty();}
2578         }
2579         public boolean containsKey(Object key) {
2580             synchronized (mutex) {return m.containsKey(key);}
2581         }
2582         public boolean containsValue(Object value) {
2583             synchronized (mutex) {return m.containsValue(value);}
2584         }
2585         public V get(Object key) {
2586             synchronized (mutex) {return m.get(key);}
2587         }
2588 
2589         public V put(K key, V value) {
2590             synchronized (mutex) {return m.put(key, value);}
2591         }
2592         public V remove(Object key) {
2593             synchronized (mutex) {return m.remove(key);}
2594         }
2595         public void putAll(Map<? extends K, ? extends V> map) {
2596             synchronized (mutex) {m.putAll(map);}
2597         }
2598         public void clear() {
2599             synchronized (mutex) {m.clear();}
2600         }
2601 
2602         private transient Set<K> keySet;
2603         private transient Set<Map.Entry<K,V>> entrySet;
2604         private transient Collection<V> values;
2605 
2606         public Set<K> keySet() {
2607             synchronized (mutex) {
2608                 if (keySet==null)
2609                     keySet = new SynchronizedSet<>(m.keySet(), mutex);
2610                 return keySet;
2611             }
2612         }
2613 
2614         public Set<Map.Entry<K,V>> entrySet() {
2615             synchronized (mutex) {
2616                 if (entrySet==null)
2617                     entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
2618                 return entrySet;
2619             }
2620         }
2621 
2622         public Collection<V> values() {
2623             synchronized (mutex) {
2624                 if (values==null)
2625                     values = new SynchronizedCollection<>(m.values(), mutex);
2626                 return values;
2627             }
2628         }
2629 
2630         public boolean equals(Object o) {
2631             if (this == o)
2632                 return true;
2633             synchronized (mutex) {return m.equals(o);}
2634         }
2635         public int hashCode() {
2636             synchronized (mutex) {return m.hashCode();}
2637         }
2638         public String toString() {
2639             synchronized (mutex) {return m.toString();}
2640         }
2641 
2642         // Override default methods in Map
2643         @Override
2644         public V getOrDefault(Object k, V defaultValue) {
2645             synchronized (mutex) {return m.getOrDefault(k, defaultValue);}
2646         }
2647         @Override
2648         public void forEach(BiConsumer<? super K, ? super V> action) {
2649             synchronized (mutex) {m.forEach(action);}
2650         }
2651         @Override
2652         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
2653             synchronized (mutex) {m.replaceAll(function);}
2654         }
2655         @Override
2656         public V putIfAbsent(K key, V value) {
2657             synchronized (mutex) {return m.putIfAbsent(key, value);}
2658         }
2659         @Override
2660         public boolean remove(Object key, Object value) {
2661             synchronized (mutex) {return m.remove(key, value);}
2662         }
2663         @Override
2664         public boolean replace(K key, V oldValue, V newValue) {
2665             synchronized (mutex) {return m.replace(key, oldValue, newValue);}
2666         }
2667         @Override
2668         public V replace(K key, V value) {
2669             synchronized (mutex) {return m.replace(key, value);}
2670         }
2671         @Override
2672         public V computeIfAbsent(K key,
2673                 Function<? super K, ? extends V> mappingFunction) {
2674             synchronized (mutex) {return m.computeIfAbsent(key, mappingFunction);}
2675         }
2676         @Override
2677         public V computeIfPresent(K key,
2678                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
2679             synchronized (mutex) {return m.computeIfPresent(key, remappingFunction);}
2680         }
2681         @Override
2682         public V compute(K key,
2683                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
2684             synchronized (mutex) {return m.compute(key, remappingFunction);}
2685         }
2686         @Override
2687         public V merge(K key, V value,
2688                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
2689             synchronized (mutex) {return m.merge(key, value, remappingFunction);}
2690         }
2691 
2692         private void writeObject(ObjectOutputStream s) throws IOException {
2693             synchronized (mutex) {s.defaultWriteObject();}
2694         }
2695     }
2696 
2697     /**
2698      * Returns a synchronized (thread-safe) sorted map backed by the specified
2699      * sorted map.  In order to guarantee serial access, it is critical that
2700      * <strong>all</strong> access to the backing sorted map is accomplished
2701      * through the returned sorted map (or its views).<p>
2702      *
2703      * It is imperative that the user manually synchronize on the returned
2704      * sorted map when iterating over any of its collection views, or the
2705      * collections views of any of its <tt>subMap</tt>, <tt>headMap</tt> or
2706      * <tt>tailMap</tt> views.
2707      * <pre>
2708      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
2709      *      ...
2710      *  Set s = m.keySet();  // Needn't be in synchronized block
2711      *      ...
2712      *  synchronized (m) {  // Synchronizing on m, not s!
2713      *      Iterator i = s.iterator(); // Must be in synchronized block
2714      *      while (i.hasNext())
2715      *          foo(i.next());
2716      *  }
2717      * </pre>
2718      * or:
2719      * <pre>
2720      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
2721      *  SortedMap m2 = m.subMap(foo, bar);
2722      *      ...
2723      *  Set s2 = m2.keySet();  // Needn't be in synchronized block
2724      *      ...
2725      *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
2726      *      Iterator i = s.iterator(); // Must be in synchronized block
2727      *      while (i.hasNext())
2728      *          foo(i.next());
2729      *  }
2730      * </pre>
2731      * Failure to follow this advice may result in non-deterministic behavior.
2732      *
2733      * <p>The returned sorted map will be serializable if the specified
2734      * sorted map is serializable.
2735      *
2736      * @param <K> the class of the map keys
2737      * @param <V> the class of the map values
2738      * @param  m the sorted map to be "wrapped" in a synchronized sorted map.
2739      * @return a synchronized view of the specified sorted map.
2740      */
2741     public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) {
2742         return new SynchronizedSortedMap<>(m);
2743     }
2744 
2745     /**
2746      * @serial include
2747      */
2748     static class SynchronizedSortedMap<K,V>
2749         extends SynchronizedMap<K,V>
2750         implements SortedMap<K,V>
2751     {
2752         private static final long serialVersionUID = -8798146769416483793L;
2753 
2754         private final SortedMap<K,V> sm;
2755 
2756         SynchronizedSortedMap(SortedMap<K,V> m) {
2757             super(m);
2758             sm = m;
2759         }
2760         SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
2761             super(m, mutex);
2762             sm = m;
2763         }
2764 
2765         public Comparator<? super K> comparator() {
2766             synchronized (mutex) {return sm.comparator();}
2767         }
2768 
2769         public SortedMap<K,V> subMap(K fromKey, K toKey) {
2770             synchronized (mutex) {
2771                 return new SynchronizedSortedMap<>(
2772                     sm.subMap(fromKey, toKey), mutex);
2773             }
2774         }
2775         public SortedMap<K,V> headMap(K toKey) {
2776             synchronized (mutex) {
2777                 return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
2778             }
2779         }
2780         public SortedMap<K,V> tailMap(K fromKey) {
2781             synchronized (mutex) {
2782                return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);
2783             }
2784         }
2785 
2786         public K firstKey() {
2787             synchronized (mutex) {return sm.firstKey();}
2788         }
2789         public K lastKey() {
2790             synchronized (mutex) {return sm.lastKey();}
2791         }
2792     }
2793 
2794     /**
2795      * Returns a synchronized (thread-safe) navigable map backed by the
2796      * specified navigable map.  In order to guarantee serial access, it is
2797      * critical that <strong>all</strong> access to the backing navigable map is
2798      * accomplished through the returned navigable map (or its views).<p>
2799      *
2800      * It is imperative that the user manually synchronize on the returned
2801      * navigable map when iterating over any of its collection views, or the
2802      * collections views of any of its {@code subMap}, {@code headMap} or
2803      * {@code tailMap} views.
2804      * <pre>
2805      *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
2806      *      ...
2807      *  Set s = m.keySet();  // Needn't be in synchronized block
2808      *      ...
2809      *  synchronized (m) {  // Synchronizing on m, not s!
2810      *      Iterator i = s.iterator(); // Must be in synchronized block
2811      *      while (i.hasNext())
2812      *          foo(i.next());
2813      *  }
2814      * </pre>
2815      * or:
2816      * <pre>
2817      *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
2818      *  NavigableMap m2 = m.subMap(foo, true, bar, false);
2819      *      ...
2820      *  Set s2 = m2.keySet();  // Needn't be in synchronized block
2821      *      ...
2822      *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
2823      *      Iterator i = s.iterator(); // Must be in synchronized block
2824      *      while (i.hasNext())
2825      *          foo(i.next());
2826      *  }
2827      * </pre>
2828      * Failure to follow this advice may result in non-deterministic behavior.
2829      *
2830      * <p>The returned navigable map will be serializable if the specified
2831      * navigable map is serializable.
2832      *
2833      * @param <K> the class of the map keys
2834      * @param <V> the class of the map values
2835      * @param  m the navigable map to be "wrapped" in a synchronized navigable
2836      *              map
2837      * @return a synchronized view of the specified navigable map.
2838      * @since 1.8
2839      */
2840     public static <K,V> NavigableMap<K,V> synchronizedNavigableMap(NavigableMap<K,V> m) {
2841         return new SynchronizedNavigableMap<>(m);
2842     }
2843 
2844     /**
2845      * A synchronized NavigableMap.
2846      *
2847      * @serial include
2848      */
2849     static class SynchronizedNavigableMap<K,V>
2850         extends SynchronizedSortedMap<K,V>
2851         implements NavigableMap<K,V>
2852     {
2853         private static final long serialVersionUID = 699392247599746807L;
2854 
2855         private final NavigableMap<K,V> nm;
2856 
2857         SynchronizedNavigableMap(NavigableMap<K,V> m) {
2858             super(m);
2859             nm = m;
2860         }
2861         SynchronizedNavigableMap(NavigableMap<K,V> m, Object mutex) {
2862             super(m, mutex);
2863             nm = m;
2864         }
2865 
2866         public Entry<K, V> lowerEntry(K key)
2867                         { synchronized (mutex) { return nm.lowerEntry(key); } }
2868         public K lowerKey(K key)
2869                           { synchronized (mutex) { return nm.lowerKey(key); } }
2870         public Entry<K, V> floorEntry(K key)
2871                         { synchronized (mutex) { return nm.floorEntry(key); } }
2872         public K floorKey(K key)
2873                           { synchronized (mutex) { return nm.floorKey(key); } }
2874         public Entry<K, V> ceilingEntry(K key)
2875                       { synchronized (mutex) { return nm.ceilingEntry(key); } }
2876         public K ceilingKey(K key)
2877                         { synchronized (mutex) { return nm.ceilingKey(key); } }
2878         public Entry<K, V> higherEntry(K key)
2879                        { synchronized (mutex) { return nm.higherEntry(key); } }
2880         public K higherKey(K key)
2881                          { synchronized (mutex) { return nm.higherKey(key); } }
2882         public Entry<K, V> firstEntry()
2883                            { synchronized (mutex) { return nm.firstEntry(); } }
2884         public Entry<K, V> lastEntry()
2885                             { synchronized (mutex) { return nm.lastEntry(); } }
2886         public Entry<K, V> pollFirstEntry()
2887                        { synchronized (mutex) { return nm.pollFirstEntry(); } }
2888         public Entry<K, V> pollLastEntry()
2889                         { synchronized (mutex) { return nm.pollLastEntry(); } }
2890 
2891         public NavigableMap<K, V> descendingMap() {
2892             synchronized (mutex) {
2893                 return
2894                     new SynchronizedNavigableMap<>(nm.descendingMap(), mutex);
2895             }
2896         }
2897 
2898         public NavigableSet<K> keySet() {
2899             return navigableKeySet();
2900         }
2901 
2902         public NavigableSet<K> navigableKeySet() {
2903             synchronized (mutex) {
2904                 return new SynchronizedNavigableSet<>(nm.navigableKeySet(), mutex);
2905             }
2906         }
2907 
2908         public NavigableSet<K> descendingKeySet() {
2909             synchronized (mutex) {
2910                 return new SynchronizedNavigableSet<>(nm.descendingKeySet(), mutex);
2911             }
2912         }
2913 
2914 
2915         public SortedMap<K,V> subMap(K fromKey, K toKey) {
2916             synchronized (mutex) {
2917                 return new SynchronizedNavigableMap<>(
2918                     nm.subMap(fromKey, true, toKey, false), mutex);
2919             }
2920         }
2921         public SortedMap<K,V> headMap(K toKey) {
2922             synchronized (mutex) {
2923                 return new SynchronizedNavigableMap<>(nm.headMap(toKey, false), mutex);
2924             }
2925         }
2926         public SortedMap<K,V> tailMap(K fromKey) {
2927             synchronized (mutex) {
2928         return new SynchronizedNavigableMap<>(nm.tailMap(fromKey, true),mutex);
2929             }
2930         }
2931 
2932         public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
2933             synchronized (mutex) {
2934                 return new SynchronizedNavigableMap<>(
2935                     nm.subMap(fromKey, fromInclusive, toKey, toInclusive), mutex);
2936             }
2937         }
2938 
2939         public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
2940             synchronized (mutex) {
2941                 return new SynchronizedNavigableMap<>(
2942                         nm.headMap(toKey, inclusive), mutex);
2943             }
2944         }
2945 
2946         public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
2947             synchronized (mutex) {
2948                 return new SynchronizedNavigableMap<>(
2949                     nm.tailMap(fromKey, inclusive), mutex);
2950             }
2951         }
2952     }
2953 
2954     // Dynamically typesafe collection wrappers
2955 
2956     /**
2957      * Returns a dynamically typesafe view of the specified collection.
2958      * Any attempt to insert an element of the wrong type will result in an
2959      * immediate {@link ClassCastException}.  Assuming a collection
2960      * contains no incorrectly typed elements prior to the time a
2961      * dynamically typesafe view is generated, and that all subsequent
2962      * access to the collection takes place through the view, it is
2963      * <i>guaranteed</i> that the collection cannot contain an incorrectly
2964      * typed element.
2965      *
2966      * <p>The generics mechanism in the language provides compile-time
2967      * (static) type checking, but it is possible to defeat this mechanism
2968      * with unchecked casts.  Usually this is not a problem, as the compiler
2969      * issues warnings on all such unchecked operations.  There are, however,
2970      * times when static type checking alone is not sufficient.  For example,
2971      * suppose a collection is passed to a third-party library and it is
2972      * imperative that the library code not corrupt the collection by
2973      * inserting an element of the wrong type.
2974      *
2975      * <p>Another use of dynamically typesafe views is debugging.  Suppose a
2976      * program fails with a {@code ClassCastException}, indicating that an
2977      * incorrectly typed element was put into a parameterized collection.
2978      * Unfortunately, the exception can occur at any time after the erroneous
2979      * element is inserted, so it typically provides little or no information
2980      * as to the real source of the problem.  If the problem is reproducible,
2981      * one can quickly determine its source by temporarily modifying the
2982      * program to wrap the collection with a dynamically typesafe view.
2983      * For example, this declaration:
2984      *  <pre> {@code
2985      *     Collection<String> c = new HashSet<>();
2986      * }</pre>
2987      * may be replaced temporarily by this one:
2988      *  <pre> {@code
2989      *     Collection<String> c = Collections.checkedCollection(
2990      *         new HashSet<>(), String.class);
2991      * }</pre>
2992      * Running the program again will cause it to fail at the point where
2993      * an incorrectly typed element is inserted into the collection, clearly
2994      * identifying the source of the problem.  Once the problem is fixed, the
2995      * modified declaration may be reverted back to the original.
2996      *
2997      * <p>The returned collection does <i>not</i> pass the hashCode and equals
2998      * operations through to the backing collection, but relies on
2999      * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
3000      * is necessary to preserve the contracts of these operations in the case
3001      * that the backing collection is a set or a list.
3002      *
3003      * <p>The returned collection will be serializable if the specified
3004      * collection is serializable.
3005      *
3006      * <p>Since {@code null} is considered to be a value of any reference
3007      * type, the returned collection permits insertion of null elements
3008      * whenever the backing collection does.
3009      *
3010      * @param <E> the class of the objects in the collection
3011      * @param c the collection for which a dynamically typesafe view is to be
3012      *          returned
3013      * @param type the type of element that {@code c} is permitted to hold
3014      * @return a dynamically typesafe view of the specified collection
3015      * @since 1.5
3016      */
3017     public static <E> Collection<E> checkedCollection(Collection<E> c,
3018                                                       Class<E> type) {
3019         return new CheckedCollection<>(c, type);
3020     }
3021 
3022     @SuppressWarnings("unchecked")
3023     static <T> T[] zeroLengthArray(Class<T> type) {
3024         return (T[]) Array.newInstance(type, 0);
3025     }
3026 
3027     /**
3028      * @serial include
3029      */
3030     static class CheckedCollection<E> implements Collection<E>, Serializable {
3031         private static final long serialVersionUID = 1578914078182001775L;
3032 
3033         final Collection<E> c;
3034         final Class<E> type;
3035 
3036         @SuppressWarnings("unchecked")
3037         E typeCheck(Object o) {
3038             if (o != null && !type.isInstance(o))
3039                 throw new ClassCastException(badElementMsg(o));
3040             return (E) o;
3041         }
3042 
3043         private String badElementMsg(Object o) {
3044             return "Attempt to insert " + o.getClass() +
3045                 " element into collection with element type " + type;
3046         }
3047 
3048         CheckedCollection(Collection<E> c, Class<E> type) {
3049             this.c = Objects.requireNonNull(c, "c");
3050             this.type = Objects.requireNonNull(type, "type");
3051         }
3052 
3053         public int size()                 { return c.size(); }
3054         public boolean isEmpty()          { return c.isEmpty(); }
3055         public boolean contains(Object o) { return c.contains(o); }
3056         public Object[] toArray()         { return c.toArray(); }
3057         public <T> T[] toArray(T[] a)     { return c.toArray(a); }
3058         public String toString()          { return c.toString(); }
3059         public boolean remove(Object o)   { return c.remove(o); }
3060         public void clear()               {        c.clear(); }
3061 
3062         public boolean containsAll(Collection<?> coll) {
3063             return c.containsAll(coll);
3064         }
3065         public boolean removeAll(Collection<?> coll) {
3066             return c.removeAll(coll);
3067         }
3068         public boolean retainAll(Collection<?> coll) {
3069             return c.retainAll(coll);
3070         }
3071 
3072         public Iterator<E> iterator() {
3073             // JDK-6363904 - unwrapped iterator could be typecast to
3074             // ListIterator with unsafe set()
3075             final Iterator<E> it = c.iterator();
3076             return new Iterator<E>() {
3077                 public boolean hasNext() { return it.hasNext(); }
3078                 public E next()          { return it.next(); }
3079                 public void remove()     {        it.remove(); }};
3080         }
3081 
3082         public boolean add(E e)          { return c.add(typeCheck(e)); }
3083 
3084         private E[] zeroLengthElementArray; // Lazily initialized
3085 
3086         private E[] zeroLengthElementArray() {
3087             return zeroLengthElementArray != null ? zeroLengthElementArray :
3088                 (zeroLengthElementArray = zeroLengthArray(type));
3089         }
3090 
3091         @SuppressWarnings("unchecked")
3092         Collection<E> checkedCopyOf(Collection<? extends E> coll) {
3093             Object[] a;
3094             try {
3095                 E[] z = zeroLengthElementArray();
3096                 a = coll.toArray(z);
3097                 // Defend against coll violating the toArray contract
3098                 if (a.getClass() != z.getClass())
3099                     a = Arrays.copyOf(a, a.length, z.getClass());
3100             } catch (ArrayStoreException ignore) {
3101                 // To get better and consistent diagnostics,
3102                 // we call typeCheck explicitly on each element.
3103                 // We call clone() to defend against coll retaining a
3104                 // reference to the returned array and storing a bad
3105                 // element into it after it has been type checked.
3106                 a = coll.toArray().clone();
3107                 for (Object o : a)
3108                     typeCheck(o);
3109             }
3110             // A slight abuse of the type system, but safe here.
3111             return (Collection<E>) Arrays.asList(a);
3112         }
3113 
3114         public boolean addAll(Collection<? extends E> coll) {
3115             // Doing things this way insulates us from concurrent changes
3116             // in the contents of coll and provides all-or-nothing
3117             // semantics (which we wouldn't get if we type-checked each
3118             // element as we added it)
3119             return c.addAll(checkedCopyOf(coll));
3120         }
3121 
3122         // Override default methods in Collection
3123         @Override
3124         public void forEach(Consumer<? super E> action) {c.forEach(action);}
3125         @Override
3126         public boolean removeIf(Predicate<? super E> filter) {
3127             return c.removeIf(filter);
3128         }
3129         @Override
3130         public Spliterator<E> spliterator() {return c.spliterator();}
3131         @Override
3132         public Stream<E> stream()           {return c.stream();}
3133         @Override
3134         public Stream<E> parallelStream()   {return c.parallelStream();}
3135     }
3136 
3137     /**
3138      * Returns a dynamically typesafe view of the specified queue.
3139      * Any attempt to insert an element of the wrong type will result in
3140      * an immediate {@link ClassCastException}.  Assuming a queue contains
3141      * no incorrectly typed elements prior to the time a dynamically typesafe
3142      * view is generated, and that all subsequent access to the queue
3143      * takes place through the view, it is <i>guaranteed</i> that the
3144      * queue cannot contain an incorrectly typed element.
3145      *
3146      * <p>A discussion of the use of dynamically typesafe views may be
3147      * found in the documentation for the {@link #checkedCollection
3148      * checkedCollection} method.
3149      *
3150      * <p>The returned queue will be serializable if the specified queue
3151      * is serializable.
3152      *
3153      * <p>Since {@code null} is considered to be a value of any reference
3154      * type, the returned queue permits insertion of {@code null} elements
3155      * whenever the backing queue does.
3156      *
3157      * @param <E> the class of the objects in the queue
3158      * @param queue the queue for which a dynamically typesafe view is to be
3159      *             returned
3160      * @param type the type of element that {@code queue} is permitted to hold
3161      * @return a dynamically typesafe view of the specified queue
3162      * @since 1.8
3163      */
3164     public static <E> Queue<E> checkedQueue(Queue<E> queue, Class<E> type) {
3165         return new CheckedQueue<>(queue, type);
3166     }
3167 
3168     /**
3169      * @serial include
3170      */
3171     static class CheckedQueue<E>
3172         extends CheckedCollection<E>
3173         implements Queue<E>, Serializable
3174     {
3175         private static final long serialVersionUID = 1433151992604707767L;
3176         final Queue<E> queue;
3177 
3178         CheckedQueue(Queue<E> queue, Class<E> elementType) {
3179             super(queue, elementType);
3180             this.queue = queue;
3181         }
3182 
3183         public E element()              {return queue.element();}
3184         public boolean equals(Object o) {return o == this || c.equals(o);}
3185         public int hashCode()           {return c.hashCode();}
3186         public E peek()                 {return queue.peek();}
3187         public E poll()                 {return queue.poll();}
3188         public E remove()               {return queue.remove();}
3189         public boolean offer(E e)       {return queue.offer(typeCheck(e));}
3190     }
3191 
3192     /**
3193      * Returns a dynamically typesafe view of the specified set.
3194      * Any attempt to insert an element of the wrong type will result in
3195      * an immediate {@link ClassCastException}.  Assuming a set contains
3196      * no incorrectly typed elements prior to the time a dynamically typesafe
3197      * view is generated, and that all subsequent access to the set
3198      * takes place through the view, it is <i>guaranteed</i> that the
3199      * set cannot contain an incorrectly typed element.
3200      *
3201      * <p>A discussion of the use of dynamically typesafe views may be
3202      * found in the documentation for the {@link #checkedCollection
3203      * checkedCollection} method.
3204      *
3205      * <p>The returned set will be serializable if the specified set is
3206      * serializable.
3207      *
3208      * <p>Since {@code null} is considered to be a value of any reference
3209      * type, the returned set permits insertion of null elements whenever
3210      * the backing set does.
3211      *
3212      * @param <E> the class of the objects in the set
3213      * @param s the set for which a dynamically typesafe view is to be
3214      *          returned
3215      * @param type the type of element that {@code s} is permitted to hold
3216      * @return a dynamically typesafe view of the specified set
3217      * @since 1.5
3218      */
3219     public static <E> Set<E> checkedSet(Set<E> s, Class<E> type) {
3220         return new CheckedSet<>(s, type);
3221     }
3222 
3223     /**
3224      * @serial include
3225      */
3226     static class CheckedSet<E> extends CheckedCollection<E>
3227                                  implements Set<E>, Serializable
3228     {
3229         private static final long serialVersionUID = 4694047833775013803L;
3230 
3231         CheckedSet(Set<E> s, Class<E> elementType) { super(s, elementType); }
3232 
3233         public boolean equals(Object o) { return o == this || c.equals(o); }
3234         public int hashCode()           { return c.hashCode(); }
3235     }
3236 
3237     /**
3238      * Returns a dynamically typesafe view of the specified sorted set.
3239      * Any attempt to insert an element of the wrong type will result in an
3240      * immediate {@link ClassCastException}.  Assuming a sorted set
3241      * contains no incorrectly typed elements prior to the time a
3242      * dynamically typesafe view is generated, and that all subsequent
3243      * access to the sorted set takes place through the view, it is
3244      * <i>guaranteed</i> that the sorted set cannot contain an incorrectly
3245      * typed element.
3246      *
3247      * <p>A discussion of the use of dynamically typesafe views may be
3248      * found in the documentation for the {@link #checkedCollection
3249      * checkedCollection} method.
3250      *
3251      * <p>The returned sorted set will be serializable if the specified sorted
3252      * set is serializable.
3253      *
3254      * <p>Since {@code null} is considered to be a value of any reference
3255      * type, the returned sorted set permits insertion of null elements
3256      * whenever the backing sorted set does.
3257      *
3258      * @param <E> the class of the objects in the set
3259      * @param s the sorted set for which a dynamically typesafe view is to be
3260      *          returned
3261      * @param type the type of element that {@code s} is permitted to hold
3262      * @return a dynamically typesafe view of the specified sorted set
3263      * @since 1.5
3264      */
3265     public static <E> SortedSet<E> checkedSortedSet(SortedSet<E> s,
3266                                                     Class<E> type) {
3267         return new CheckedSortedSet<>(s, type);
3268     }
3269 
3270     /**
3271      * @serial include
3272      */
3273     static class CheckedSortedSet<E> extends CheckedSet<E>
3274         implements SortedSet<E>, Serializable
3275     {
3276         private static final long serialVersionUID = 1599911165492914959L;
3277 
3278         private final SortedSet<E> ss;
3279 
3280         CheckedSortedSet(SortedSet<E> s, Class<E> type) {
3281             super(s, type);
3282             ss = s;
3283         }
3284 
3285         public Comparator<? super E> comparator() { return ss.comparator(); }
3286         public E first()                   { return ss.first(); }
3287         public E last()                    { return ss.last(); }
3288 
3289         public SortedSet<E> subSet(E fromElement, E toElement) {
3290             return checkedSortedSet(ss.subSet(fromElement,toElement), type);
3291         }
3292         public SortedSet<E> headSet(E toElement) {
3293             return checkedSortedSet(ss.headSet(toElement), type);
3294         }
3295         public SortedSet<E> tailSet(E fromElement) {
3296             return checkedSortedSet(ss.tailSet(fromElement), type);
3297         }
3298     }
3299 
3300 /**
3301      * Returns a dynamically typesafe view of the specified navigable set.
3302      * Any attempt to insert an element of the wrong type will result in an
3303      * immediate {@link ClassCastException}.  Assuming a navigable set
3304      * contains no incorrectly typed elements prior to the time a
3305      * dynamically typesafe view is generated, and that all subsequent
3306      * access to the navigable set takes place through the view, it is
3307      * <em>guaranteed</em> that the navigable set cannot contain an incorrectly
3308      * typed element.
3309      *
3310      * <p>A discussion of the use of dynamically typesafe views may be
3311      * found in the documentation for the {@link #checkedCollection
3312      * checkedCollection} method.
3313      *
3314      * <p>The returned navigable set will be serializable if the specified
3315      * navigable set is serializable.
3316      *
3317      * <p>Since {@code null} is considered to be a value of any reference
3318      * type, the returned navigable set permits insertion of null elements
3319      * whenever the backing sorted set does.
3320      *
3321      * @param <E> the class of the objects in the set
3322      * @param s the navigable set for which a dynamically typesafe view is to be
3323      *          returned
3324      * @param type the type of element that {@code s} is permitted to hold
3325      * @return a dynamically typesafe view of the specified navigable set
3326      * @since 1.8
3327      */
3328     public static <E> NavigableSet<E> checkedNavigableSet(NavigableSet<E> s,
3329                                                     Class<E> type) {
3330         return new CheckedNavigableSet<>(s, type);
3331     }
3332 
3333     /**
3334      * @serial include
3335      */
3336     static class CheckedNavigableSet<E> extends CheckedSortedSet<E>
3337         implements NavigableSet<E>, Serializable
3338     {
3339         private static final long serialVersionUID = -5429120189805438922L;
3340 
3341         private final NavigableSet<E> ns;
3342 
3343         CheckedNavigableSet(NavigableSet<E> s, Class<E> type) {
3344             super(s, type);
3345             ns = s;
3346         }
3347 
3348         public E lower(E e)                             { return ns.lower(e); }
3349         public E floor(E e)                             { return ns.floor(e); }
3350         public E ceiling(E e)                         { return ns.ceiling(e); }
3351         public E higher(E e)                           { return ns.higher(e); }
3352         public E pollFirst()                         { return ns.pollFirst(); }
3353         public E pollLast()                            {return ns.pollLast(); }
3354         public NavigableSet<E> descendingSet()
3355                       { return checkedNavigableSet(ns.descendingSet(), type); }
3356         public Iterator<E> descendingIterator()
3357             {return checkedNavigableSet(ns.descendingSet(), type).iterator(); }
3358 
3359         public NavigableSet<E> subSet(E fromElement, E toElement) {
3360             return checkedNavigableSet(ns.subSet(fromElement, true, toElement, false), type);
3361         }
3362         public NavigableSet<E> headSet(E toElement) {
3363             return checkedNavigableSet(ns.headSet(toElement, false), type);
3364         }
3365         public NavigableSet<E> tailSet(E fromElement) {
3366             return checkedNavigableSet(ns.tailSet(fromElement, true), type);
3367         }
3368 
3369         public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
3370             return checkedNavigableSet(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), type);
3371         }
3372 
3373         public NavigableSet<E> headSet(E toElement, boolean inclusive) {
3374             return checkedNavigableSet(ns.headSet(toElement, inclusive), type);
3375         }
3376 
3377         public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
3378             return checkedNavigableSet(ns.tailSet(fromElement, inclusive), type);
3379         }
3380     }
3381 
3382     /**
3383      * Returns a dynamically typesafe view of the specified list.
3384      * Any attempt to insert an element of the wrong type will result in
3385      * an immediate {@link ClassCastException}.  Assuming a list contains
3386      * no incorrectly typed elements prior to the time a dynamically typesafe
3387      * view is generated, and that all subsequent access to the list
3388      * takes place through the view, it is <i>guaranteed</i> that the
3389      * list cannot contain an incorrectly typed element.
3390      *
3391      * <p>A discussion of the use of dynamically typesafe views may be
3392      * found in the documentation for the {@link #checkedCollection
3393      * checkedCollection} method.
3394      *
3395      * <p>The returned list will be serializable if the specified list
3396      * is serializable.
3397      *
3398      * <p>Since {@code null} is considered to be a value of any reference
3399      * type, the returned list permits insertion of null elements whenever
3400      * the backing list does.
3401      *
3402      * @param <E> the class of the objects in the list
3403      * @param list the list for which a dynamically typesafe view is to be
3404      *             returned
3405      * @param type the type of element that {@code list} is permitted to hold
3406      * @return a dynamically typesafe view of the specified list
3407      * @since 1.5
3408      */
3409     public static <E> List<E> checkedList(List<E> list, Class<E> type) {
3410         return (list instanceof RandomAccess ?
3411                 new CheckedRandomAccessList<>(list, type) :
3412                 new CheckedList<>(list, type));
3413     }
3414 
3415     /**
3416      * @serial include
3417      */
3418     static class CheckedList<E>
3419         extends CheckedCollection<E>
3420         implements List<E>
3421     {
3422         private static final long serialVersionUID = 65247728283967356L;
3423         final List<E> list;
3424 
3425         CheckedList(List<E> list, Class<E> type) {
3426             super(list, type);
3427             this.list = list;
3428         }
3429 
3430         public boolean equals(Object o)  { return o == this || list.equals(o); }
3431         public int hashCode()            { return list.hashCode(); }
3432         public E get(int index)          { return list.get(index); }
3433         public E remove(int index)       { return list.remove(index); }
3434         public int indexOf(Object o)     { return list.indexOf(o); }
3435         public int lastIndexOf(Object o) { return list.lastIndexOf(o); }
3436 
3437         public E set(int index, E element) {
3438             return list.set(index, typeCheck(element));
3439         }
3440 
3441         public void add(int index, E element) {
3442             list.add(index, typeCheck(element));
3443         }
3444 
3445         public boolean addAll(int index, Collection<? extends E> c) {
3446             return list.addAll(index, checkedCopyOf(c));
3447         }
3448         public ListIterator<E> listIterator()   { return listIterator(0); }
3449 
3450         public ListIterator<E> listIterator(final int index) {
3451             final ListIterator<E> i = list.listIterator(index);
3452 
3453             return new ListIterator<E>() {
3454                 public boolean hasNext()     { return i.hasNext(); }
3455                 public E next()              { return i.next(); }
3456                 public boolean hasPrevious() { return i.hasPrevious(); }
3457                 public E previous()          { return i.previous(); }
3458                 public int nextIndex()       { return i.nextIndex(); }
3459                 public int previousIndex()   { return i.previousIndex(); }
3460                 public void remove()         {        i.remove(); }
3461 
3462                 public void set(E e) {
3463                     i.set(typeCheck(e));
3464                 }
3465 
3466                 public void add(E e) {
3467                     i.add(typeCheck(e));
3468                 }
3469 
3470                 @Override
3471                 public void forEachRemaining(Consumer<? super E> action) {
3472                     i.forEachRemaining(action);
3473                 }
3474             };
3475         }
3476 
3477         public List<E> subList(int fromIndex, int toIndex) {
3478             return new CheckedList<>(list.subList(fromIndex, toIndex), type);
3479         }
3480 
3481         /**
3482          * {@inheritDoc}
3483          *
3484          * @throws ClassCastException if the class of an element returned by the
3485          *         operator prevents it from being added to this collection. The
3486          *         exception may be thrown after some elements of the list have
3487          *         already been replaced.
3488          */
3489         @Override
3490         public void replaceAll(UnaryOperator<E> operator) {
3491             Objects.requireNonNull(operator);
3492             list.replaceAll(e -> typeCheck(operator.apply(e)));
3493         }
3494 
3495         @Override
3496         public void sort(Comparator<? super E> c) {
3497             list.sort(c);
3498         }
3499     }
3500 
3501     /**
3502      * @serial include
3503      */
3504     static class CheckedRandomAccessList<E> extends CheckedList<E>
3505                                             implements RandomAccess
3506     {
3507         private static final long serialVersionUID = 1638200125423088369L;
3508 
3509         CheckedRandomAccessList(List<E> list, Class<E> type) {
3510             super(list, type);
3511         }
3512 
3513         public List<E> subList(int fromIndex, int toIndex) {
3514             return new CheckedRandomAccessList<>(
3515                     list.subList(fromIndex, toIndex), type);
3516         }
3517     }
3518 
3519     /**
3520      * Returns a dynamically typesafe view of the specified map.
3521      * Any attempt to insert a mapping whose key or value have the wrong
3522      * type will result in an immediate {@link ClassCastException}.
3523      * Similarly, any attempt to modify the value currently associated with
3524      * a key will result in an immediate {@link ClassCastException},
3525      * whether the modification is attempted directly through the map
3526      * itself, or through a {@link Map.Entry} instance obtained from the
3527      * map's {@link Map#entrySet() entry set} view.
3528      *
3529      * <p>Assuming a map contains no incorrectly typed keys or values
3530      * prior to the time a dynamically typesafe view is generated, and
3531      * that all subsequent access to the map takes place through the view
3532      * (or one of its collection views), it is <i>guaranteed</i> that the
3533      * map cannot contain an incorrectly typed key or value.
3534      *
3535      * <p>A discussion of the use of dynamically typesafe views may be
3536      * found in the documentation for the {@link #checkedCollection
3537      * checkedCollection} method.
3538      *
3539      * <p>The returned map will be serializable if the specified map is
3540      * serializable.
3541      *
3542      * <p>Since {@code null} is considered to be a value of any reference
3543      * type, the returned map permits insertion of null keys or values
3544      * whenever the backing map does.
3545      *
3546      * @param <K> the class of the map keys
3547      * @param <V> the class of the map values
3548      * @param m the map for which a dynamically typesafe view is to be
3549      *          returned
3550      * @param keyType the type of key that {@code m} is permitted to hold
3551      * @param valueType the type of value that {@code m} is permitted to hold
3552      * @return a dynamically typesafe view of the specified map
3553      * @since 1.5
3554      */
3555     public static <K, V> Map<K, V> checkedMap(Map<K, V> m,
3556                                               Class<K> keyType,
3557                                               Class<V> valueType) {
3558         return new CheckedMap<>(m, keyType, valueType);
3559     }
3560 
3561 
3562     /**
3563      * @serial include
3564      */
3565     private static class CheckedMap<K,V>
3566         implements Map<K,V>, Serializable
3567     {
3568         private static final long serialVersionUID = 5742860141034234728L;
3569 
3570         private final Map<K, V> m;
3571         final Class<K> keyType;
3572         final Class<V> valueType;
3573 
3574         private void typeCheck(Object key, Object value) {
3575             if (key != null && !keyType.isInstance(key))
3576                 throw new ClassCastException(badKeyMsg(key));
3577 
3578             if (value != null && !valueType.isInstance(value))
3579                 throw new ClassCastException(badValueMsg(value));
3580         }
3581 
3582         private BiFunction<? super K, ? super V, ? extends V> typeCheck(
3583                 BiFunction<? super K, ? super V, ? extends V> func) {
3584             Objects.requireNonNull(func);
3585             return (k, v) -> {
3586                 V newValue = func.apply(k, v);
3587                 typeCheck(k, newValue);
3588                 return newValue;
3589             };
3590         }
3591 
3592         private String badKeyMsg(Object key) {
3593             return "Attempt to insert " + key.getClass() +
3594                     " key into map with key type " + keyType;
3595         }
3596 
3597         private String badValueMsg(Object value) {
3598             return "Attempt to insert " + value.getClass() +
3599                     " value into map with value type " + valueType;
3600         }
3601 
3602         CheckedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) {
3603             this.m = Objects.requireNonNull(m);
3604             this.keyType = Objects.requireNonNull(keyType);
3605             this.valueType = Objects.requireNonNull(valueType);
3606         }
3607 
3608         public int size()                      { return m.size(); }
3609         public boolean isEmpty()               { return m.isEmpty(); }
3610         public boolean containsKey(Object key) { return m.containsKey(key); }
3611         public boolean containsValue(Object v) { return m.containsValue(v); }
3612         public V get(Object key)               { return m.get(key); }
3613         public V remove(Object key)            { return m.remove(key); }
3614         public void clear()                    { m.clear(); }
3615         public Set<K> keySet()                 { return m.keySet(); }
3616         public Collection<V> values()          { return m.values(); }
3617         public boolean equals(Object o)        { return o == this || m.equals(o); }
3618         public int hashCode()                  { return m.hashCode(); }
3619         public String toString()               { return m.toString(); }
3620 
3621         public V put(K key, V value) {
3622             typeCheck(key, value);
3623             return m.put(key, value);
3624         }
3625 
3626         @SuppressWarnings("unchecked")
3627         public void putAll(Map<? extends K, ? extends V> t) {
3628             // Satisfy the following goals:
3629             // - good diagnostics in case of type mismatch
3630             // - all-or-nothing semantics
3631             // - protection from malicious t
3632             // - correct behavior if t is a concurrent map
3633             Object[] entries = t.entrySet().toArray();
3634             List<Map.Entry<K,V>> checked = new ArrayList<>(entries.length);
3635             for (Object o : entries) {
3636                 Map.Entry<?,?> e = (Map.Entry<?,?>) o;
3637                 Object k = e.getKey();
3638                 Object v = e.getValue();
3639                 typeCheck(k, v);
3640                 checked.add(
3641                         new AbstractMap.SimpleImmutableEntry<>((K)k, (V)v));
3642             }
3643             for (Map.Entry<K,V> e : checked)
3644                 m.put(e.getKey(), e.getValue());
3645         }
3646 
3647         private transient Set<Map.Entry<K,V>> entrySet;
3648 
3649         public Set<Map.Entry<K,V>> entrySet() {
3650             if (entrySet==null)
3651                 entrySet = new CheckedEntrySet<>(m.entrySet(), valueType);
3652             return entrySet;
3653         }
3654 
3655         // Override default methods in Map
3656         @Override
3657         public void forEach(BiConsumer<? super K, ? super V> action) {
3658             m.forEach(action);
3659         }
3660 
3661         @Override
3662         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
3663             m.replaceAll(typeCheck(function));
3664         }
3665 
3666         @Override
3667         public V putIfAbsent(K key, V value) {
3668             typeCheck(key, value);
3669             return m.putIfAbsent(key, value);
3670         }
3671 
3672         @Override
3673         public boolean remove(Object key, Object value) {
3674             return m.remove(key, value);
3675         }
3676 
3677         @Override
3678         public boolean replace(K key, V oldValue, V newValue) {
3679             typeCheck(key, newValue);
3680             return m.replace(key, oldValue, newValue);
3681         }
3682 
3683         @Override
3684         public V replace(K key, V value) {
3685             typeCheck(key, value);
3686             return m.replace(key, value);
3687         }
3688 
3689         @Override
3690         public V computeIfAbsent(K key,
3691                 Function<? super K, ? extends V> mappingFunction) {
3692             Objects.requireNonNull(mappingFunction);
3693             return m.computeIfAbsent(key, k -> {
3694                 V value = mappingFunction.apply(k);
3695                 typeCheck(k, value);
3696                 return value;
3697             });
3698         }
3699 
3700         @Override
3701         public V computeIfPresent(K key,
3702                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
3703             return m.computeIfPresent(key, typeCheck(remappingFunction));
3704         }
3705 
3706         @Override
3707         public V compute(K key,
3708                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
3709             return m.compute(key, typeCheck(remappingFunction));
3710         }
3711 
3712         @Override
3713         public V merge(K key, V value,
3714                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
3715             Objects.requireNonNull(remappingFunction);
3716             return m.merge(key, value, (v1, v2) -> {
3717                 V newValue = remappingFunction.apply(v1, v2);
3718                 typeCheck(null, newValue);
3719                 return newValue;
3720             });
3721         }
3722 
3723         /**
3724          * We need this class in addition to CheckedSet as Map.Entry permits
3725          * modification of the backing Map via the setValue operation.  This
3726          * class is subtle: there are many possible attacks that must be
3727          * thwarted.
3728          *
3729          * @serial exclude
3730          */
3731         static class CheckedEntrySet<K,V> implements Set<Map.Entry<K,V>> {
3732             private final Set<Map.Entry<K,V>> s;
3733             private final Class<V> valueType;
3734 
3735             CheckedEntrySet(Set<Map.Entry<K, V>> s, Class<V> valueType) {
3736                 this.s = s;
3737                 this.valueType = valueType;
3738             }
3739 
3740             public int size()        { return s.size(); }
3741             public boolean isEmpty() { return s.isEmpty(); }
3742             public String toString() { return s.toString(); }
3743             public int hashCode()    { return s.hashCode(); }
3744             public void clear()      {        s.clear(); }
3745 
3746             public boolean add(Map.Entry<K, V> e) {
3747                 throw new UnsupportedOperationException();
3748             }
3749             public boolean addAll(Collection<? extends Map.Entry<K, V>> coll) {
3750                 throw new UnsupportedOperationException();
3751             }
3752 
3753             public Iterator<Map.Entry<K,V>> iterator() {
3754                 final Iterator<Map.Entry<K, V>> i = s.iterator();
3755                 final Class<V> valueType = this.valueType;
3756 
3757                 return new Iterator<Map.Entry<K,V>>() {
3758                     public boolean hasNext() { return i.hasNext(); }
3759                     public void remove()     { i.remove(); }
3760 
3761                     public Map.Entry<K,V> next() {
3762                         return checkedEntry(i.next(), valueType);
3763                     }
3764                 };
3765             }
3766 
3767             @SuppressWarnings("unchecked")
3768             public Object[] toArray() {
3769                 Object[] source = s.toArray();
3770 
3771                 /*
3772                  * Ensure that we don't get an ArrayStoreException even if
3773                  * s.toArray returns an array of something other than Object
3774                  */
3775                 Object[] dest = (CheckedEntry.class.isInstance(
3776                     source.getClass().getComponentType()) ? source :
3777                                  new Object[source.length]);
3778 
3779                 for (int i = 0; i < source.length; i++)
3780                     dest[i] = checkedEntry((Map.Entry<K,V>)source[i],
3781                                            valueType);
3782                 return dest;
3783             }
3784 
3785             @SuppressWarnings("unchecked")
3786             public <T> T[] toArray(T[] a) {
3787                 // We don't pass a to s.toArray, to avoid window of
3788                 // vulnerability wherein an unscrupulous multithreaded client
3789                 // could get his hands on raw (unwrapped) Entries from s.
3790                 T[] arr = s.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
3791 
3792                 for (int i=0; i<arr.length; i++)
3793                     arr[i] = (T) checkedEntry((Map.Entry<K,V>)arr[i],
3794                                               valueType);
3795                 if (arr.length > a.length)
3796                     return arr;
3797 
3798                 System.arraycopy(arr, 0, a, 0, arr.length);
3799                 if (a.length > arr.length)
3800                     a[arr.length] = null;
3801                 return a;
3802             }
3803 
3804             /**
3805              * This method is overridden to protect the backing set against
3806              * an object with a nefarious equals function that senses
3807              * that the equality-candidate is Map.Entry and calls its
3808              * setValue method.
3809              */
3810             public boolean contains(Object o) {
3811                 if (!(o instanceof Map.Entry))
3812                     return false;
3813                 Map.Entry<?,?> e = (Map.Entry<?,?>) o;
3814                 return s.contains(
3815                     (e instanceof CheckedEntry) ? e : checkedEntry(e, valueType));
3816             }
3817 
3818             /**
3819              * The bulk collection methods are overridden to protect
3820              * against an unscrupulous collection whose contains(Object o)
3821              * method senses when o is a Map.Entry, and calls o.setValue.
3822              */
3823             public boolean containsAll(Collection<?> c) {
3824                 for (Object o : c)
3825                     if (!contains(o)) // Invokes safe contains() above
3826                         return false;
3827                 return true;
3828             }
3829 
3830             public boolean remove(Object o) {
3831                 if (!(o instanceof Map.Entry))
3832                     return false;
3833                 return s.remove(new AbstractMap.SimpleImmutableEntry
3834                                 <>((Map.Entry<?,?>)o));
3835             }
3836 
3837             public boolean removeAll(Collection<?> c) {
3838                 return batchRemove(c, false);
3839             }
3840             public boolean retainAll(Collection<?> c) {
3841                 return batchRemove(c, true);
3842             }
3843             private boolean batchRemove(Collection<?> c, boolean complement) {
3844                 Objects.requireNonNull(c);
3845                 boolean modified = false;
3846                 Iterator<Map.Entry<K,V>> it = iterator();
3847                 while (it.hasNext()) {
3848                     if (c.contains(it.next()) != complement) {
3849                         it.remove();
3850                         modified = true;
3851                     }
3852                 }
3853                 return modified;
3854             }
3855 
3856             public boolean equals(Object o) {
3857                 if (o == this)
3858                     return true;
3859                 if (!(o instanceof Set))
3860                     return false;
3861                 Set<?> that = (Set<?>) o;
3862                 return that.size() == s.size()
3863                     && containsAll(that); // Invokes safe containsAll() above
3864             }
3865 
3866             static <K,V,T> CheckedEntry<K,V,T> checkedEntry(Map.Entry<K,V> e,
3867                                                             Class<T> valueType) {
3868                 return new CheckedEntry<>(e, valueType);
3869             }
3870 
3871             /**
3872              * This "wrapper class" serves two purposes: it prevents
3873              * the client from modifying the backing Map, by short-circuiting
3874              * the setValue method, and it protects the backing Map against
3875              * an ill-behaved Map.Entry that attempts to modify another
3876              * Map.Entry when asked to perform an equality check.
3877              */
3878             private static class CheckedEntry<K,V,T> implements Map.Entry<K,V> {
3879                 private final Map.Entry<K, V> e;
3880                 private final Class<T> valueType;
3881 
3882                 CheckedEntry(Map.Entry<K, V> e, Class<T> valueType) {
3883                     this.e = Objects.requireNonNull(e);
3884                     this.valueType = Objects.requireNonNull(valueType);
3885                 }
3886 
3887                 public K getKey()        { return e.getKey(); }
3888                 public V getValue()      { return e.getValue(); }
3889                 public int hashCode()    { return e.hashCode(); }
3890                 public String toString() { return e.toString(); }
3891 
3892                 public V setValue(V value) {
3893                     if (value != null && !valueType.isInstance(value))
3894                         throw new ClassCastException(badValueMsg(value));
3895                     return e.setValue(value);
3896                 }
3897 
3898                 private String badValueMsg(Object value) {
3899                     return "Attempt to insert " + value.getClass() +
3900                         " value into map with value type " + valueType;
3901                 }
3902 
3903                 public boolean equals(Object o) {
3904                     if (o == this)
3905                         return true;
3906                     if (!(o instanceof Map.Entry))
3907                         return false;
3908                     return e.equals(new AbstractMap.SimpleImmutableEntry
3909                                     <>((Map.Entry<?,?>)o));
3910                 }
3911             }
3912         }
3913     }
3914 
3915     /**
3916      * Returns a dynamically typesafe view of the specified sorted map.
3917      * Any attempt to insert a mapping whose key or value have the wrong
3918      * type will result in an immediate {@link ClassCastException}.
3919      * Similarly, any attempt to modify the value currently associated with
3920      * a key will result in an immediate {@link ClassCastException},
3921      * whether the modification is attempted directly through the map
3922      * itself, or through a {@link Map.Entry} instance obtained from the
3923      * map's {@link Map#entrySet() entry set} view.
3924      *
3925      * <p>Assuming a map contains no incorrectly typed keys or values
3926      * prior to the time a dynamically typesafe view is generated, and
3927      * that all subsequent access to the map takes place through the view
3928      * (or one of its collection views), it is <i>guaranteed</i> that the
3929      * map cannot contain an incorrectly typed key or value.
3930      *
3931      * <p>A discussion of the use of dynamically typesafe views may be
3932      * found in the documentation for the {@link #checkedCollection
3933      * checkedCollection} method.
3934      *
3935      * <p>The returned map will be serializable if the specified map is
3936      * serializable.
3937      *
3938      * <p>Since {@code null} is considered to be a value of any reference
3939      * type, the returned map permits insertion of null keys or values
3940      * whenever the backing map does.
3941      *
3942      * @param <K> the class of the map keys
3943      * @param <V> the class of the map values
3944      * @param m the map for which a dynamically typesafe view is to be
3945      *          returned
3946      * @param keyType the type of key that {@code m} is permitted to hold
3947      * @param valueType the type of value that {@code m} is permitted to hold
3948      * @return a dynamically typesafe view of the specified map
3949      * @since 1.5
3950      */
3951     public static <K,V> SortedMap<K,V> checkedSortedMap(SortedMap<K, V> m,
3952                                                         Class<K> keyType,
3953                                                         Class<V> valueType) {
3954         return new CheckedSortedMap<>(m, keyType, valueType);
3955     }
3956 
3957     /**
3958      * @serial include
3959      */
3960     static class CheckedSortedMap<K,V> extends CheckedMap<K,V>
3961         implements SortedMap<K,V>, Serializable
3962     {
3963         private static final long serialVersionUID = 1599671320688067438L;
3964 
3965         private final SortedMap<K, V> sm;
3966 
3967         CheckedSortedMap(SortedMap<K, V> m,
3968                          Class<K> keyType, Class<V> valueType) {
3969             super(m, keyType, valueType);
3970             sm = m;
3971         }
3972 
3973         public Comparator<? super K> comparator() { return sm.comparator(); }
3974         public K firstKey()                       { return sm.firstKey(); }
3975         public K lastKey()                        { return sm.lastKey(); }
3976 
3977         public SortedMap<K,V> subMap(K fromKey, K toKey) {
3978             return checkedSortedMap(sm.subMap(fromKey, toKey),
3979                                     keyType, valueType);
3980         }
3981         public SortedMap<K,V> headMap(K toKey) {
3982             return checkedSortedMap(sm.headMap(toKey), keyType, valueType);
3983         }
3984         public SortedMap<K,V> tailMap(K fromKey) {
3985             return checkedSortedMap(sm.tailMap(fromKey), keyType, valueType);
3986         }
3987     }
3988 
3989     /**
3990      * Returns a dynamically typesafe view of the specified navigable map.
3991      * Any attempt to insert a mapping whose key or value have the wrong
3992      * type will result in an immediate {@link ClassCastException}.
3993      * Similarly, any attempt to modify the value currently associated with
3994      * a key will result in an immediate {@link ClassCastException},
3995      * whether the modification is attempted directly through the map
3996      * itself, or through a {@link Map.Entry} instance obtained from the
3997      * map's {@link Map#entrySet() entry set} view.
3998      *
3999      * <p>Assuming a map contains no incorrectly typed keys or values
4000      * prior to the time a dynamically typesafe view is generated, and
4001      * that all subsequent access to the map takes place through the view
4002      * (or one of its collection views), it is <em>guaranteed</em> that the
4003      * map cannot contain an incorrectly typed key or value.
4004      *
4005      * <p>A discussion of the use of dynamically typesafe views may be
4006      * found in the documentation for the {@link #checkedCollection
4007      * checkedCollection} method.
4008      *
4009      * <p>The returned map will be serializable if the specified map is
4010      * serializable.
4011      *
4012      * <p>Since {@code null} is considered to be a value of any reference
4013      * type, the returned map permits insertion of null keys or values
4014      * whenever the backing map does.
4015      *
4016      * @param <K> type of map keys
4017      * @param <V> type of map values
4018      * @param m the map for which a dynamically typesafe view is to be
4019      *          returned
4020      * @param keyType the type of key that {@code m} is permitted to hold
4021      * @param valueType the type of value that {@code m} is permitted to hold
4022      * @return a dynamically typesafe view of the specified map
4023      * @since 1.8
4024      */
4025     public static <K,V> NavigableMap<K,V> checkedNavigableMap(NavigableMap<K, V> m,
4026                                                         Class<K> keyType,
4027                                                         Class<V> valueType) {
4028         return new CheckedNavigableMap<>(m, keyType, valueType);
4029     }
4030 
4031     /**
4032      * @serial include
4033      */
4034     static class CheckedNavigableMap<K,V> extends CheckedSortedMap<K,V>
4035         implements NavigableMap<K,V>, Serializable
4036     {
4037         private static final long serialVersionUID = -4852462692372534096L;
4038 
4039         private final NavigableMap<K, V> nm;
4040 
4041         CheckedNavigableMap(NavigableMap<K, V> m,
4042                          Class<K> keyType, Class<V> valueType) {
4043             super(m, keyType, valueType);
4044             nm = m;
4045         }
4046 
4047         public Comparator<? super K> comparator()   { return nm.comparator(); }
4048         public K firstKey()                           { return nm.firstKey(); }
4049         public K lastKey()                             { return nm.lastKey(); }
4050 
4051         public Entry<K, V> lowerEntry(K key) {
4052             Entry<K,V> lower = nm.lowerEntry(key);
4053             return (null != lower)
4054                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(lower, valueType)
4055                 : null;
4056         }
4057 
4058         public K lowerKey(K key)                   { return nm.lowerKey(key); }
4059 
4060         public Entry<K, V> floorEntry(K key) {
4061             Entry<K,V> floor = nm.floorEntry(key);
4062             return (null != floor)
4063                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(floor, valueType)
4064                 : null;
4065         }
4066 
4067         public K floorKey(K key)                   { return nm.floorKey(key); }
4068 
4069         public Entry<K, V> ceilingEntry(K key) {
4070             Entry<K,V> ceiling = nm.ceilingEntry(key);
4071             return (null != ceiling)
4072                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(ceiling, valueType)
4073                 : null;
4074         }
4075 
4076         public K ceilingKey(K key)               { return nm.ceilingKey(key); }
4077 
4078         public Entry<K, V> higherEntry(K key) {
4079             Entry<K,V> higher = nm.higherEntry(key);
4080             return (null != higher)
4081                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(higher, valueType)
4082                 : null;
4083         }
4084 
4085         public K higherKey(K key)                 { return nm.higherKey(key); }
4086 
4087         public Entry<K, V> firstEntry() {
4088             Entry<K,V> first = nm.firstEntry();
4089             return (null != first)
4090                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(first, valueType)
4091                 : null;
4092         }
4093 
4094         public Entry<K, V> lastEntry() {
4095             Entry<K,V> last = nm.lastEntry();
4096             return (null != last)
4097                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(last, valueType)
4098                 : null;
4099         }
4100 
4101         public Entry<K, V> pollFirstEntry() {
4102             Entry<K,V> entry = nm.pollFirstEntry();
4103             return (null == entry)
4104                 ? null
4105                 : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
4106         }
4107 
4108         public Entry<K, V> pollLastEntry() {
4109             Entry<K,V> entry = nm.pollLastEntry();
4110             return (null == entry)
4111                 ? null
4112                 : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
4113         }
4114 
4115         public NavigableMap<K, V> descendingMap() {
4116             return checkedNavigableMap(nm.descendingMap(), keyType, valueType);
4117         }
4118 
4119         public NavigableSet<K> keySet() {
4120             return navigableKeySet();
4121         }
4122 
4123         public NavigableSet<K> navigableKeySet() {
4124             return checkedNavigableSet(nm.navigableKeySet(), keyType);
4125         }
4126 
4127         public NavigableSet<K> descendingKeySet() {
4128             return checkedNavigableSet(nm.descendingKeySet(), keyType);
4129         }
4130 
4131         @Override
4132         public NavigableMap<K,V> subMap(K fromKey, K toKey) {
4133             return checkedNavigableMap(nm.subMap(fromKey, true, toKey, false),
4134                                     keyType, valueType);
4135         }
4136 
4137         @Override
4138         public NavigableMap<K,V> headMap(K toKey) {
4139             return checkedNavigableMap(nm.headMap(toKey, false), keyType, valueType);
4140         }
4141 
4142         @Override
4143         public NavigableMap<K,V> tailMap(K fromKey) {
4144             return checkedNavigableMap(nm.tailMap(fromKey, true), keyType, valueType);
4145         }
4146 
4147         public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
4148             return checkedNavigableMap(nm.subMap(fromKey, fromInclusive, toKey, toInclusive), keyType, valueType);
4149         }
4150 
4151         public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
4152             return checkedNavigableMap(nm.headMap(toKey, inclusive), keyType, valueType);
4153         }
4154 
4155         public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
4156             return checkedNavigableMap(nm.tailMap(fromKey, inclusive), keyType, valueType);
4157         }
4158     }
4159 
4160     // Empty collections
4161 
4162     /**
4163      * Returns an iterator that has no elements.  More precisely,
4164      *
4165      * <ul>
4166      * <li>{@link Iterator#hasNext hasNext} always returns {@code
4167      * false}.</li>
4168      * <li>{@link Iterator#next next} always throws {@link
4169      * NoSuchElementException}.</li>
4170      * <li>{@link Iterator#remove remove} always throws {@link
4171      * IllegalStateException}.</li>
4172      * </ul>
4173      *
4174      * <p>Implementations of this method are permitted, but not
4175      * required, to return the same object from multiple invocations.
4176      *
4177      * @param <T> type of elements, if there were any, in the iterator
4178      * @return an empty iterator
4179      * @since 1.7
4180      */
4181     @SuppressWarnings("unchecked")
4182     public static <T> Iterator<T> emptyIterator() {
4183         return (Iterator<T>) EmptyIterator.EMPTY_ITERATOR;
4184     }
4185 
4186     private static class EmptyIterator<E> implements Iterator<E> {
4187         static final EmptyIterator<Object> EMPTY_ITERATOR
4188             = new EmptyIterator<>();
4189 
4190         public boolean hasNext() { return false; }
4191         public E next() { throw new NoSuchElementException(); }
4192         public void remove() { throw new IllegalStateException(); }
4193         @Override
4194         public void forEachRemaining(Consumer<? super E> action) {
4195             Objects.requireNonNull(action);
4196         }
4197     }
4198 
4199     /**
4200      * Returns a list iterator that has no elements.  More precisely,
4201      *
4202      * <ul>
4203      * <li>{@link Iterator#hasNext hasNext} and {@link
4204      * ListIterator#hasPrevious hasPrevious} always return {@code
4205      * false}.</li>
4206      * <li>{@link Iterator#next next} and {@link ListIterator#previous
4207      * previous} always throw {@link NoSuchElementException}.</li>
4208      * <li>{@link Iterator#remove remove} and {@link ListIterator#set
4209      * set} always throw {@link IllegalStateException}.</li>
4210      * <li>{@link ListIterator#add add} always throws {@link
4211      * UnsupportedOperationException}.</li>
4212      * <li>{@link ListIterator#nextIndex nextIndex} always returns
4213      * {@code 0}.</li>
4214      * <li>{@link ListIterator#previousIndex previousIndex} always
4215      * returns {@code -1}.</li>
4216      * </ul>
4217      *
4218      * <p>Implementations of this method are permitted, but not
4219      * required, to return the same object from multiple invocations.
4220      *
4221      * @param <T> type of elements, if there were any, in the iterator
4222      * @return an empty list iterator
4223      * @since 1.7
4224      */
4225     @SuppressWarnings("unchecked")
4226     public static <T> ListIterator<T> emptyListIterator() {
4227         return (ListIterator<T>) EmptyListIterator.EMPTY_ITERATOR;
4228     }
4229 
4230     private static class EmptyListIterator<E>
4231         extends EmptyIterator<E>
4232         implements ListIterator<E>
4233     {
4234         static final EmptyListIterator<Object> EMPTY_ITERATOR
4235             = new EmptyListIterator<>();
4236 
4237         public boolean hasPrevious() { return false; }
4238         public E previous() { throw new NoSuchElementException(); }
4239         public int nextIndex()     { return 0; }
4240         public int previousIndex() { return -1; }
4241         public void set(E e) { throw new IllegalStateException(); }
4242         public void add(E e) { throw new UnsupportedOperationException(); }
4243     }
4244 
4245     /**
4246      * Returns an enumeration that has no elements.  More precisely,
4247      *
4248      * <ul>
4249      * <li>{@link Enumeration#hasMoreElements hasMoreElements} always
4250      * returns {@code false}.</li>
4251      * <li> {@link Enumeration#nextElement nextElement} always throws
4252      * {@link NoSuchElementException}.</li>
4253      * </ul>
4254      *
4255      * <p>Implementations of this method are permitted, but not
4256      * required, to return the same object from multiple invocations.
4257      *
4258      * @param  <T> the class of the objects in the enumeration
4259      * @return an empty enumeration
4260      * @since 1.7
4261      */
4262     @SuppressWarnings("unchecked")
4263     public static <T> Enumeration<T> emptyEnumeration() {
4264         return (Enumeration<T>) EmptyEnumeration.EMPTY_ENUMERATION;
4265     }
4266 
4267     private static class EmptyEnumeration<E> implements Enumeration<E> {
4268         static final EmptyEnumeration<Object> EMPTY_ENUMERATION
4269             = new EmptyEnumeration<>();
4270 
4271         public boolean hasMoreElements() { return false; }
4272         public E nextElement() { throw new NoSuchElementException(); }
4273     }
4274 
4275     /**
4276      * The empty set (immutable).  This set is serializable.
4277      *
4278      * @see #emptySet()
4279      */
4280     @SuppressWarnings("rawtypes")
4281     public static final Set EMPTY_SET = new EmptySet<>();
4282 
4283     /**
4284      * Returns an empty set (immutable).  This set is serializable.
4285      * Unlike the like-named field, this method is parameterized.
4286      *
4287      * <p>This example illustrates the type-safe way to obtain an empty set:
4288      * <pre>
4289      *     Set&lt;String&gt; s = Collections.emptySet();
4290      * </pre>
4291      * @implNote Implementations of this method need not create a separate
4292      * {@code Set} object for each call.  Using this method is likely to have
4293      * comparable cost to using the like-named field.  (Unlike this method, the
4294      * field does not provide type safety.)
4295      *
4296      * @param  <T> the class of the objects in the set
4297      * @return the empty set
4298      *
4299      * @see #EMPTY_SET
4300      * @since 1.5
4301      */
4302     @SuppressWarnings("unchecked")
4303     public static final <T> Set<T> emptySet() {
4304         return (Set<T>) EMPTY_SET;
4305     }
4306 
4307     /**
4308      * @serial include
4309      */
4310     private static class EmptySet<E>
4311         extends AbstractSet<E>
4312         implements Serializable
4313     {
4314         private static final long serialVersionUID = 1582296315990362920L;
4315 
4316         public Iterator<E> iterator() { return emptyIterator(); }
4317 
4318         public int size() {return 0;}
4319         public boolean isEmpty() {return true;}
4320 
4321         public boolean contains(Object obj) {return false;}
4322         public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
4323 
4324         public Object[] toArray() { return new Object[0]; }
4325 
4326         public <T> T[] toArray(T[] a) {
4327             if (a.length > 0)
4328                 a[0] = null;
4329             return a;
4330         }
4331 
4332         // Override default methods in Collection
4333         @Override
4334         public void forEach(Consumer<? super E> action) {
4335             Objects.requireNonNull(action);
4336         }
4337         @Override
4338         public boolean removeIf(Predicate<? super E> filter) {
4339             Objects.requireNonNull(filter);
4340             return false;
4341         }
4342         @Override
4343         public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
4344 
4345         // Preserves singleton property
4346         private Object readResolve() {
4347             return EMPTY_SET;
4348         }
4349     }
4350 
4351     /**
4352      * Returns an empty sorted set (immutable).  This set is serializable.
4353      *
4354      * <p>This example illustrates the type-safe way to obtain an empty
4355      * sorted set:
4356      * <pre> {@code
4357      *     SortedSet<String> s = Collections.emptySortedSet();
4358      * }</pre>
4359      *
4360      * @implNote Implementations of this method need not create a separate
4361      * {@code SortedSet} object for each call.
4362      *
4363      * @param <E> type of elements, if there were any, in the set
4364      * @return the empty sorted set
4365      * @since 1.8
4366      */
4367     @SuppressWarnings("unchecked")
4368     public static <E> SortedSet<E> emptySortedSet() {
4369         return (SortedSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
4370     }
4371 
4372     /**
4373      * Returns an empty navigable set (immutable).  This set is serializable.
4374      *
4375      * <p>This example illustrates the type-safe way to obtain an empty
4376      * navigable set:
4377      * <pre> {@code
4378      *     NavigableSet<String> s = Collections.emptyNavigableSet();
4379      * }</pre>
4380      *
4381      * @implNote Implementations of this method need not
4382      * create a separate {@code NavigableSet} object for each call.
4383      *
4384      * @param <E> type of elements, if there were any, in the set
4385      * @return the empty navigable set
4386      * @since 1.8
4387      */
4388     @SuppressWarnings("unchecked")
4389     public static <E> NavigableSet<E> emptyNavigableSet() {
4390         return (NavigableSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
4391     }
4392 
4393     /**
4394      * The empty list (immutable).  This list is serializable.
4395      *
4396      * @see #emptyList()
4397      */
4398     @SuppressWarnings("rawtypes")
4399     public static final List EMPTY_LIST = new EmptyList<>();
4400 
4401     /**
4402      * Returns an empty list (immutable).  This list is serializable.
4403      *
4404      * <p>This example illustrates the type-safe way to obtain an empty list:
4405      * <pre>
4406      *     List&lt;String&gt; s = Collections.emptyList();
4407      * </pre>
4408      *
4409      * @implNote
4410      * Implementations of this method need not create a separate <tt>List</tt>
4411      * object for each call.   Using this method is likely to have comparable
4412      * cost to using the like-named field.  (Unlike this method, the field does
4413      * not provide type safety.)
4414      *
4415      * @param <T> type of elements, if there were any, in the list
4416      * @return an empty immutable list
4417      *
4418      * @see #EMPTY_LIST
4419      * @since 1.5
4420      */
4421     @SuppressWarnings("unchecked")
4422     public static final <T> List<T> emptyList() {
4423         return (List<T>) EMPTY_LIST;
4424     }
4425 
4426     /**
4427      * @serial include
4428      */
4429     private static class EmptyList<E>
4430         extends AbstractList<E>
4431         implements RandomAccess, Serializable {
4432         private static final long serialVersionUID = 8842843931221139166L;
4433 
4434         public Iterator<E> iterator() {
4435             return emptyIterator();
4436         }
4437         public ListIterator<E> listIterator() {
4438             return emptyListIterator();
4439         }
4440 
4441         public int size() {return 0;}
4442         public boolean isEmpty() {return true;}
4443 
4444         public boolean contains(Object obj) {return false;}
4445         public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
4446 
4447         public Object[] toArray() { return new Object[0]; }
4448 
4449         public <T> T[] toArray(T[] a) {
4450             if (a.length > 0)
4451                 a[0] = null;
4452             return a;
4453         }
4454 
4455         public E get(int index) {
4456             throw new IndexOutOfBoundsException("Index: "+index);
4457         }
4458 
4459         public boolean equals(Object o) {
4460             return (o instanceof List) && ((List<?>)o).isEmpty();
4461         }
4462 
4463         public int hashCode() { return 1; }
4464 
4465         @Override
4466         public boolean removeIf(Predicate<? super E> filter) {
4467             Objects.requireNonNull(filter);
4468             return false;
4469         }
4470         @Override
4471         public void replaceAll(UnaryOperator<E> operator) {
4472             Objects.requireNonNull(operator);
4473         }
4474         @Override
4475         public void sort(Comparator<? super E> c) {
4476         }
4477 
4478         // Override default methods in Collection
4479         @Override
4480         public void forEach(Consumer<? super E> action) {
4481             Objects.requireNonNull(action);
4482         }
4483 
4484         @Override
4485         public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
4486 
4487         // Preserves singleton property
4488         private Object readResolve() {
4489             return EMPTY_LIST;
4490         }
4491     }
4492 
4493     /**
4494      * The empty map (immutable).  This map is serializable.
4495      *
4496      * @see #emptyMap()
4497      * @since 1.3
4498      */
4499     @SuppressWarnings("rawtypes")
4500     public static final Map EMPTY_MAP = new EmptyMap<>();
4501 
4502     /**
4503      * Returns an empty map (immutable).  This map is serializable.
4504      *
4505      * <p>This example illustrates the type-safe way to obtain an empty map:
4506      * <pre>
4507      *     Map&lt;String, Date&gt; s = Collections.emptyMap();
4508      * </pre>
4509      * @implNote Implementations of this method need not create a separate
4510      * {@code Map} object for each call.  Using this method is likely to have
4511      * comparable cost to using the like-named field.  (Unlike this method, the
4512      * field does not provide type safety.)
4513      *
4514      * @param <K> the class of the map keys
4515      * @param <V> the class of the map values
4516      * @return an empty map
4517      * @see #EMPTY_MAP
4518      * @since 1.5
4519      */
4520     @SuppressWarnings("unchecked")
4521     public static final <K,V> Map<K,V> emptyMap() {
4522         return (Map<K,V>) EMPTY_MAP;
4523     }
4524 
4525     /**
4526      * Returns an empty sorted map (immutable).  This map is serializable.
4527      *
4528      * <p>This example illustrates the type-safe way to obtain an empty map:
4529      * <pre> {@code
4530      *     SortedMap<String, Date> s = Collections.emptySortedMap();
4531      * }</pre>
4532      *
4533      * @implNote Implementations of this method need not create a separate
4534      * {@code SortedMap} object for each call.
4535      *
4536      * @param <K> the class of the map keys
4537      * @param <V> the class of the map values
4538      * @return an empty sorted map
4539      * @since 1.8
4540      */
4541     @SuppressWarnings("unchecked")
4542     public static final <K,V> SortedMap<K,V> emptySortedMap() {
4543         return (SortedMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
4544     }
4545 
4546     /**
4547      * Returns an empty navigable map (immutable).  This map is serializable.
4548      *
4549      * <p>This example illustrates the type-safe way to obtain an empty map:
4550      * <pre> {@code
4551      *     NavigableMap<String, Date> s = Collections.emptyNavigableMap();
4552      * }</pre>
4553      *
4554      * @implNote Implementations of this method need not create a separate
4555      * {@code NavigableMap} object for each call.
4556      *
4557      * @param <K> the class of the map keys
4558      * @param <V> the class of the map values
4559      * @return an empty navigable map
4560      * @since 1.8
4561      */
4562     @SuppressWarnings("unchecked")
4563     public static final <K,V> NavigableMap<K,V> emptyNavigableMap() {
4564         return (NavigableMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
4565     }
4566 
4567     /**
4568      * @serial include
4569      */
4570     private static class EmptyMap<K,V>
4571         extends AbstractMap<K,V>
4572         implements Serializable
4573     {
4574         private static final long serialVersionUID = 6428348081105594320L;
4575 
4576         public int size()                          {return 0;}
4577         public boolean isEmpty()                   {return true;}
4578         public boolean containsKey(Object key)     {return false;}
4579         public boolean containsValue(Object value) {return false;}
4580         public V get(Object key)                   {return null;}
4581         public Set<K> keySet()                     {return emptySet();}
4582         public Collection<V> values()              {return emptySet();}
4583         public Set<Map.Entry<K,V>> entrySet()      {return emptySet();}
4584 
4585         public boolean equals(Object o) {
4586             return (o instanceof Map) && ((Map<?,?>)o).isEmpty();
4587         }
4588 
4589         public int hashCode()                      {return 0;}
4590 
4591         // Override default methods in Map
4592         @Override
4593         @SuppressWarnings("unchecked")
4594         public V getOrDefault(Object k, V defaultValue) {
4595             return defaultValue;
4596         }
4597 
4598         @Override
4599         public void forEach(BiConsumer<? super K, ? super V> action) {
4600             Objects.requireNonNull(action);
4601         }
4602 
4603         @Override
4604         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
4605             Objects.requireNonNull(function);
4606         }
4607 
4608         @Override
4609         public V putIfAbsent(K key, V value) {
4610             throw new UnsupportedOperationException();
4611         }
4612 
4613         @Override
4614         public boolean remove(Object key, Object value) {
4615             throw new UnsupportedOperationException();
4616         }
4617 
4618         @Override
4619         public boolean replace(K key, V oldValue, V newValue) {
4620             throw new UnsupportedOperationException();
4621         }
4622 
4623         @Override
4624         public V replace(K key, V value) {
4625             throw new UnsupportedOperationException();
4626         }
4627 
4628         @Override
4629         public V computeIfAbsent(K key,
4630                 Function<? super K, ? extends V> mappingFunction) {
4631             throw new UnsupportedOperationException();
4632         }
4633 
4634         @Override
4635         public V computeIfPresent(K key,
4636                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4637             throw new UnsupportedOperationException();
4638         }
4639 
4640         @Override
4641         public V compute(K key,
4642                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4643             throw new UnsupportedOperationException();
4644         }
4645 
4646         @Override
4647         public V merge(K key, V value,
4648                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
4649             throw new UnsupportedOperationException();
4650         }
4651 
4652         // Preserves singleton property
4653         private Object readResolve() {
4654             return EMPTY_MAP;
4655         }
4656     }
4657 
4658     // Singleton collections
4659 
4660     /**
4661      * Returns an immutable set containing only the specified object.
4662      * The returned set is serializable.
4663      *
4664      * @param  <T> the class of the objects in the set
4665      * @param o the sole object to be stored in the returned set.
4666      * @return an immutable set containing only the specified object.
4667      */
4668     public static <T> Set<T> singleton(T o) {
4669         return new SingletonSet<>(o);
4670     }
4671 
4672     static <E> Iterator<E> singletonIterator(final E e) {
4673         return new Iterator<E>() {
4674             private boolean hasNext = true;
4675             public boolean hasNext() {
4676                 return hasNext;
4677             }
4678             public E next() {
4679                 if (hasNext) {
4680                     hasNext = false;
4681                     return e;
4682                 }
4683                 throw new NoSuchElementException();
4684             }
4685             public void remove() {
4686                 throw new UnsupportedOperationException();
4687             }
4688             @Override
4689             public void forEachRemaining(Consumer<? super E> action) {
4690                 Objects.requireNonNull(action);
4691                 if (hasNext) {
4692                     action.accept(e);
4693                     hasNext = false;
4694                 }
4695             }
4696         };
4697     }
4698 
4699     /**
4700      * Creates a {@code Spliterator} with only the specified element
4701      *
4702      * @param <T> Type of elements
4703      * @return A singleton {@code Spliterator}
4704      */
4705     static <T> Spliterator<T> singletonSpliterator(final T element) {
4706         return new Spliterator<T>() {
4707             long est = 1;
4708 
4709             @Override
4710             public Spliterator<T> trySplit() {
4711                 return null;
4712             }
4713 
4714             @Override
4715             public boolean tryAdvance(Consumer<? super T> consumer) {
4716                 Objects.requireNonNull(consumer);
4717                 if (est > 0) {
4718                     est--;
4719                     consumer.accept(element);
4720                     return true;
4721                 }
4722                 return false;
4723             }
4724 
4725             @Override
4726             public void forEachRemaining(Consumer<? super T> consumer) {
4727                 tryAdvance(consumer);
4728             }
4729 
4730             @Override
4731             public long estimateSize() {
4732                 return est;
4733             }
4734 
4735             @Override
4736             public int characteristics() {
4737                 int value = (element != null) ? Spliterator.NONNULL : 0;
4738 
4739                 return value | Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.IMMUTABLE |
4740                        Spliterator.DISTINCT | Spliterator.ORDERED;
4741             }
4742         };
4743     }
4744 
4745     /**
4746      * @serial include
4747      */
4748     private static class SingletonSet<E>
4749         extends AbstractSet<E>
4750         implements Serializable
4751     {
4752         private static final long serialVersionUID = 3193687207550431679L;
4753 
4754         private final E element;
4755 
4756         SingletonSet(E e) {element = e;}
4757 
4758         public Iterator<E> iterator() {
4759             return singletonIterator(element);
4760         }
4761 
4762         public int size() {return 1;}
4763 
4764         public boolean contains(Object o) {return eq(o, element);}
4765 
4766         // Override default methods for Collection
4767         @Override
4768         public void forEach(Consumer<? super E> action) {
4769             action.accept(element);
4770         }
4771         @Override
4772         public Spliterator<E> spliterator() {
4773             return singletonSpliterator(element);
4774         }
4775         @Override
4776         public boolean removeIf(Predicate<? super E> filter) {
4777             throw new UnsupportedOperationException();
4778         }
4779     }
4780 
4781     /**
4782      * Returns an immutable list containing only the specified object.
4783      * The returned list is serializable.
4784      *
4785      * @param  <T> the class of the objects in the list
4786      * @param o the sole object to be stored in the returned list.
4787      * @return an immutable list containing only the specified object.
4788      * @since 1.3
4789      */
4790     public static <T> List<T> singletonList(T o) {
4791         return new SingletonList<>(o);
4792     }
4793 
4794     /**
4795      * @serial include
4796      */
4797     private static class SingletonList<E>
4798         extends AbstractList<E>
4799         implements RandomAccess, Serializable {
4800 
4801         private static final long serialVersionUID = 3093736618740652951L;
4802 
4803         private final E element;
4804 
4805         SingletonList(E obj)                {element = obj;}
4806 
4807         public Iterator<E> iterator() {
4808             return singletonIterator(element);
4809         }
4810 
4811         public int size()                   {return 1;}
4812 
4813         public boolean contains(Object obj) {return eq(obj, element);}
4814 
4815         public E get(int index) {
4816             if (index != 0)
4817               throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
4818             return element;
4819         }
4820 
4821         // Override default methods for Collection
4822         @Override
4823         public void forEach(Consumer<? super E> action) {
4824             action.accept(element);
4825         }
4826         @Override
4827         public boolean removeIf(Predicate<? super E> filter) {
4828             throw new UnsupportedOperationException();
4829         }
4830         @Override
4831         public void replaceAll(UnaryOperator<E> operator) {
4832             throw new UnsupportedOperationException();
4833         }
4834         @Override
4835         public void sort(Comparator<? super E> c) {
4836         }
4837         @Override
4838         public Spliterator<E> spliterator() {
4839             return singletonSpliterator(element);
4840         }
4841     }
4842 
4843     /**
4844      * Returns an immutable map, mapping only the specified key to the
4845      * specified value.  The returned map is serializable.
4846      *
4847      * @param <K> the class of the map keys
4848      * @param <V> the class of the map values
4849      * @param key the sole key to be stored in the returned map.
4850      * @param value the value to which the returned map maps <tt>key</tt>.
4851      * @return an immutable map containing only the specified key-value
4852      *         mapping.
4853      * @since 1.3
4854      */
4855     public static <K,V> Map<K,V> singletonMap(K key, V value) {
4856         return new SingletonMap<>(key, value);
4857     }
4858 
4859     /**
4860      * @serial include
4861      */
4862     private static class SingletonMap<K,V>
4863           extends AbstractMap<K,V>
4864           implements Serializable {
4865         private static final long serialVersionUID = -6979724477215052911L;
4866 
4867         private final K k;
4868         private final V v;
4869 
4870         SingletonMap(K key, V value) {
4871             k = key;
4872             v = value;
4873         }
4874 
4875         public int size()                                           {return 1;}
4876         public boolean isEmpty()                                {return false;}
4877         public boolean containsKey(Object key)             {return eq(key, k);}
4878         public boolean containsValue(Object value)       {return eq(value, v);}
4879         public V get(Object key)              {return (eq(key, k) ? v : null);}
4880 
4881         private transient Set<K> keySet;
4882         private transient Set<Map.Entry<K,V>> entrySet;
4883         private transient Collection<V> values;
4884 
4885         public Set<K> keySet() {
4886             if (keySet==null)
4887                 keySet = singleton(k);
4888             return keySet;
4889         }
4890 
4891         public Set<Map.Entry<K,V>> entrySet() {
4892             if (entrySet==null)
4893                 entrySet = Collections.<Map.Entry<K,V>>singleton(
4894                     new SimpleImmutableEntry<>(k, v));
4895             return entrySet;
4896         }
4897 
4898         public Collection<V> values() {
4899             if (values==null)
4900                 values = singleton(v);
4901             return values;
4902         }
4903 
4904         // Override default methods in Map
4905         @Override
4906         public V getOrDefault(Object key, V defaultValue) {
4907             return eq(key, k) ? v : defaultValue;
4908         }
4909 
4910         @Override
4911         public void forEach(BiConsumer<? super K, ? super V> action) {
4912             action.accept(k, v);
4913         }
4914 
4915         @Override
4916         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
4917             throw new UnsupportedOperationException();
4918         }
4919 
4920         @Override
4921         public V putIfAbsent(K key, V value) {
4922             throw new UnsupportedOperationException();
4923         }
4924 
4925         @Override
4926         public boolean remove(Object key, Object value) {
4927             throw new UnsupportedOperationException();
4928         }
4929 
4930         @Override
4931         public boolean replace(K key, V oldValue, V newValue) {
4932             throw new UnsupportedOperationException();
4933         }
4934 
4935         @Override
4936         public V replace(K key, V value) {
4937             throw new UnsupportedOperationException();
4938         }
4939 
4940         @Override
4941         public V computeIfAbsent(K key,
4942                 Function<? super K, ? extends V> mappingFunction) {
4943             throw new UnsupportedOperationException();
4944         }
4945 
4946         @Override
4947         public V computeIfPresent(K key,
4948                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4949             throw new UnsupportedOperationException();
4950         }
4951 
4952         @Override
4953         public V compute(K key,
4954                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4955             throw new UnsupportedOperationException();
4956         }
4957 
4958         @Override
4959         public V merge(K key, V value,
4960                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
4961             throw new UnsupportedOperationException();
4962         }
4963     }
4964 
4965     // Miscellaneous
4966 
4967     /**
4968      * Returns an immutable list consisting of <tt>n</tt> copies of the
4969      * specified object.  The newly allocated data object is tiny (it contains
4970      * a single reference to the data object).  This method is useful in
4971      * combination with the <tt>List.addAll</tt> method to grow lists.
4972      * The returned list is serializable.
4973      *
4974      * @param  <T> the class of the object to copy and of the objects
4975      *         in the returned list.
4976      * @param  n the number of elements in the returned list.
4977      * @param  o the element to appear repeatedly in the returned list.
4978      * @return an immutable list consisting of <tt>n</tt> copies of the
4979      *         specified object.
4980      * @throws IllegalArgumentException if {@code n < 0}
4981      * @see    List#addAll(Collection)
4982      * @see    List#addAll(int, Collection)
4983      */
4984     public static <T> List<T> nCopies(int n, T o) {
4985         if (n < 0)
4986             throw new IllegalArgumentException("List length = " + n);
4987         return new CopiesList<>(n, o);
4988     }
4989 
4990     /**
4991      * @serial include
4992      */
4993     private static class CopiesList<E>
4994         extends AbstractList<E>
4995         implements RandomAccess, Serializable
4996     {
4997         private static final long serialVersionUID = 2739099268398711800L;
4998 
4999         final int n;
5000         final E element;
5001 
5002         CopiesList(int n, E e) {
5003             assert n >= 0;
5004             this.n = n;
5005             element = e;
5006         }
5007 
5008         public int size() {
5009             return n;
5010         }
5011 
5012         public boolean contains(Object obj) {
5013             return n != 0 && eq(obj, element);
5014         }
5015 
5016         public int indexOf(Object o) {
5017             return contains(o) ? 0 : -1;
5018         }
5019 
5020         public int lastIndexOf(Object o) {
5021             return contains(o) ? n - 1 : -1;
5022         }
5023 
5024         public E get(int index) {
5025             if (index < 0 || index >= n)
5026                 throw new IndexOutOfBoundsException("Index: "+index+
5027                                                     ", Size: "+n);
5028             return element;
5029         }
5030 
5031         public Object[] toArray() {
5032             final Object[] a = new Object[n];
5033             if (element != null)
5034                 Arrays.fill(a, 0, n, element);
5035             return a;
5036         }
5037 
5038         @SuppressWarnings("unchecked")
5039         public <T> T[] toArray(T[] a) {
5040             final int n = this.n;
5041             if (a.length < n) {
5042                 a = (T[])java.lang.reflect.Array
5043                     .newInstance(a.getClass().getComponentType(), n);
5044                 if (element != null)
5045                     Arrays.fill(a, 0, n, element);
5046             } else {
5047                 Arrays.fill(a, 0, n, element);
5048                 if (a.length > n)
5049                     a[n] = null;
5050             }
5051             return a;
5052         }
5053 
5054         public List<E> subList(int fromIndex, int toIndex) {
5055             if (fromIndex < 0)
5056                 throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
5057             if (toIndex > n)
5058                 throw new IndexOutOfBoundsException("toIndex = " + toIndex);
5059             if (fromIndex > toIndex)
5060                 throw new IllegalArgumentException("fromIndex(" + fromIndex +
5061                                                    ") > toIndex(" + toIndex + ")");
5062             return new CopiesList<>(toIndex - fromIndex, element);
5063         }
5064 
5065         @Override
5066         public int hashCode() {
5067             if (n == 0) return 1;
5068             // hashCode of n repeating elements is 31^n + elementHash * Sum(31^k, k = 0..n-1)
5069             // this implementation completes in O(log(n)) steps taking advantage of
5070             // 31^(2*n) = (31^n)^2 and Sum(31^k, k = 0..(2*n-1)) = Sum(31^k, k = 0..n-1) * (31^n + 1)
5071             int pow = 31;
5072             int sum = 1;
5073             for (int i = Integer.numberOfLeadingZeros(n) + 1; i < Integer.SIZE; i++) {
5074                 sum *= pow + 1;
5075                 pow *= pow;
5076                 if ((n << i) < 0) {
5077                     pow *= 31;
5078                     sum = sum * 31 + 1;
5079                 }
5080             }
5081             return pow + sum * (element == null ? 0 : element.hashCode());
5082         }
5083 
5084         @Override
5085         public boolean equals(Object o) {
5086             if (o == this)
5087                 return true;
5088             if (o instanceof CopiesList) {
5089                 CopiesList<?> other = (CopiesList<?>) o;
5090                 return n == other.n && (n == 0 || eq(element, other.element));
5091             }
5092             if (!(o instanceof List))
5093                 return false;
5094 
5095             int remaining = n;
5096             E e = element;
5097             Iterator<?> itr = ((List<?>) o).iterator();
5098             if (e == null) {
5099                 while (itr.hasNext() && remaining-- > 0) {
5100                     if (itr.next() != null)
5101                         return false;
5102                 }
5103             } else {
5104                 while (itr.hasNext() && remaining-- > 0) {
5105                     if (!e.equals(itr.next()))
5106                         return false;
5107                 }
5108             }
5109             return remaining == 0 && !itr.hasNext();
5110         }
5111 
5112         // Override default methods in Collection
5113         @Override
5114         public Stream<E> stream() {
5115             return IntStream.range(0, n).mapToObj(i -> element);
5116         }
5117 
5118         @Override
5119         public Stream<E> parallelStream() {
5120             return IntStream.range(0, n).parallel().mapToObj(i -> element);
5121         }
5122 
5123         @Override
5124         public Spliterator<E> spliterator() {
5125             return stream().spliterator();
5126         }
5127 
5128         private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
5129             ois.defaultReadObject();
5130             SharedSecrets.getJavaOISAccess().checkArray(ois, Object[].class, n);
5131         }
5132     }
5133 
5134     /**
5135      * Returns a comparator that imposes the reverse of the <em>natural
5136      * ordering</em> on a collection of objects that implement the
5137      * {@code Comparable} interface.  (The natural ordering is the ordering
5138      * imposed by the objects' own {@code compareTo} method.)  This enables a
5139      * simple idiom for sorting (or maintaining) collections (or arrays) of
5140      * objects that implement the {@code Comparable} interface in
5141      * reverse-natural-order.  For example, suppose {@code a} is an array of
5142      * strings. Then: <pre>
5143      *          Arrays.sort(a, Collections.reverseOrder());
5144      * </pre> sorts the array in reverse-lexicographic (alphabetical) order.<p>
5145      *
5146      * The returned comparator is serializable.
5147      *
5148      * @param  <T> the class of the objects compared by the comparator
5149      * @return A comparator that imposes the reverse of the <i>natural
5150      *         ordering</i> on a collection of objects that implement
5151      *         the <tt>Comparable</tt> interface.
5152      * @see Comparable
5153      */
5154     @SuppressWarnings("unchecked")
5155     public static <T> Comparator<T> reverseOrder() {
5156         return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
5157     }
5158 
5159     /**
5160      * @serial include
5161      */
5162     private static class ReverseComparator
5163         implements Comparator<Comparable<Object>>, Serializable {
5164 
5165         private static final long serialVersionUID = 7207038068494060240L;
5166 
5167         static final ReverseComparator REVERSE_ORDER
5168             = new ReverseComparator();
5169 
5170         public int compare(Comparable<Object> c1, Comparable<Object> c2) {
5171             return c2.compareTo(c1);
5172         }
5173 
5174         private Object readResolve() { return Collections.reverseOrder(); }
5175 
5176         @Override
5177         public Comparator<Comparable<Object>> reversed() {
5178             return Comparator.naturalOrder();
5179         }
5180     }
5181 
5182     /**
5183      * Returns a comparator that imposes the reverse ordering of the specified
5184      * comparator.  If the specified comparator is {@code null}, this method is
5185      * equivalent to {@link #reverseOrder()} (in other words, it returns a
5186      * comparator that imposes the reverse of the <em>natural ordering</em> on
5187      * a collection of objects that implement the Comparable interface).
5188      *
5189      * <p>The returned comparator is serializable (assuming the specified
5190      * comparator is also serializable or {@code null}).
5191      *
5192      * @param <T> the class of the objects compared by the comparator
5193      * @param cmp a comparator who's ordering is to be reversed by the returned
5194      * comparator or {@code null}
5195      * @return A comparator that imposes the reverse ordering of the
5196      *         specified comparator.
5197      * @since 1.5
5198      */
5199     public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
5200         if (cmp == null)
5201             return reverseOrder();
5202 
5203         if (cmp instanceof ReverseComparator2)
5204             return ((ReverseComparator2<T>)cmp).cmp;
5205 
5206         return new ReverseComparator2<>(cmp);
5207     }
5208 
5209     /**
5210      * @serial include
5211      */
5212     private static class ReverseComparator2<T> implements Comparator<T>,
5213         Serializable
5214     {
5215         private static final long serialVersionUID = 4374092139857L;
5216 
5217         /**
5218          * The comparator specified in the static factory.  This will never
5219          * be null, as the static factory returns a ReverseComparator
5220          * instance if its argument is null.
5221          *
5222          * @serial
5223          */
5224         final Comparator<T> cmp;
5225 
5226         ReverseComparator2(Comparator<T> cmp) {
5227             assert cmp != null;
5228             this.cmp = cmp;
5229         }
5230 
5231         public int compare(T t1, T t2) {
5232             return cmp.compare(t2, t1);
5233         }
5234 
5235         public boolean equals(Object o) {
5236             return (o == this) ||
5237                 (o instanceof ReverseComparator2 &&
5238                  cmp.equals(((ReverseComparator2)o).cmp));
5239         }
5240 
5241         public int hashCode() {
5242             return cmp.hashCode() ^ Integer.MIN_VALUE;
5243         }
5244 
5245         @Override
5246         public Comparator<T> reversed() {
5247             return cmp;
5248         }
5249     }
5250 
5251     /**
5252      * Returns an enumeration over the specified collection.  This provides
5253      * interoperability with legacy APIs that require an enumeration
5254      * as input.
5255      *
5256      * @param  <T> the class of the objects in the collection
5257      * @param c the collection for which an enumeration is to be returned.
5258      * @return an enumeration over the specified collection.
5259      * @see Enumeration
5260      */
5261     public static <T> Enumeration<T> enumeration(final Collection<T> c) {
5262         return new Enumeration<T>() {
5263             private final Iterator<T> i = c.iterator();
5264 
5265             public boolean hasMoreElements() {
5266                 return i.hasNext();
5267             }
5268 
5269             public T nextElement() {
5270                 return i.next();
5271             }
5272         };
5273     }
5274 
5275     /**
5276      * Returns an array list containing the elements returned by the
5277      * specified enumeration in the order they are returned by the
5278      * enumeration.  This method provides interoperability between
5279      * legacy APIs that return enumerations and new APIs that require
5280      * collections.
5281      *
5282      * @param <T> the class of the objects returned by the enumeration
5283      * @param e enumeration providing elements for the returned
5284      *          array list
5285      * @return an array list containing the elements returned
5286      *         by the specified enumeration.
5287      * @since 1.4
5288      * @see Enumeration
5289      * @see ArrayList
5290      */
5291     public static <T> ArrayList<T> list(Enumeration<T> e) {
5292         ArrayList<T> l = new ArrayList<>();
5293         while (e.hasMoreElements())
5294             l.add(e.nextElement());
5295         return l;
5296     }
5297 
5298     /**
5299      * Returns true if the specified arguments are equal, or both null.
5300      *
5301      * NB: Do not replace with Object.equals until JDK-8015417 is resolved.
5302      */
5303     static boolean eq(Object o1, Object o2) {
5304         return o1==null ? o2==null : o1.equals(o2);
5305     }
5306 
5307     /**
5308      * Returns the number of elements in the specified collection equal to the
5309      * specified object.  More formally, returns the number of elements
5310      * <tt>e</tt> in the collection such that
5311      * <tt>(o == null ? e == null : o.equals(e))</tt>.
5312      *
5313      * @param c the collection in which to determine the frequency
5314      *     of <tt>o</tt>
5315      * @param o the object whose frequency is to be determined
5316      * @return the number of elements in {@code c} equal to {@code o}
5317      * @throws NullPointerException if <tt>c</tt> is null
5318      * @since 1.5
5319      */
5320     public static int frequency(Collection<?> c, Object o) {
5321         int result = 0;
5322         if (o == null) {
5323             for (Object e : c)
5324                 if (e == null)
5325                     result++;
5326         } else {
5327             for (Object e : c)
5328                 if (o.equals(e))
5329                     result++;
5330         }
5331         return result;
5332     }
5333 
5334     /**
5335      * Returns {@code true} if the two specified collections have no
5336      * elements in common.
5337      *
5338      * <p>Care must be exercised if this method is used on collections that
5339      * do not comply with the general contract for {@code Collection}.
5340      * Implementations may elect to iterate over either collection and test
5341      * for containment in the other collection (or to perform any equivalent
5342      * computation).  If either collection uses a nonstandard equality test
5343      * (as does a {@link SortedSet} whose ordering is not <em>compatible with
5344      * equals</em>, or the key set of an {@link IdentityHashMap}), both
5345      * collections must use the same nonstandard equality test, or the
5346      * result of this method is undefined.
5347      *
5348      * <p>Care must also be exercised when using collections that have
5349      * restrictions on the elements that they may contain. Collection
5350      * implementations are allowed to throw exceptions for any operation
5351      * involving elements they deem ineligible. For absolute safety the
5352      * specified collections should contain only elements which are
5353      * eligible elements for both collections.
5354      *
5355      * <p>Note that it is permissible to pass the same collection in both
5356      * parameters, in which case the method will return {@code true} if and
5357      * only if the collection is empty.
5358      *
5359      * @param c1 a collection
5360      * @param c2 a collection
5361      * @return {@code true} if the two specified collections have no
5362      * elements in common.
5363      * @throws NullPointerException if either collection is {@code null}.
5364      * @throws NullPointerException if one collection contains a {@code null}
5365      * element and {@code null} is not an eligible element for the other collection.
5366      * (<a href="Collection.html#optional-restrictions">optional</a>)
5367      * @throws ClassCastException if one collection contains an element that is
5368      * of a type which is ineligible for the other collection.
5369      * (<a href="Collection.html#optional-restrictions">optional</a>)
5370      * @since 1.5
5371      */
5372     public static boolean disjoint(Collection<?> c1, Collection<?> c2) {
5373         // The collection to be used for contains(). Preference is given to
5374         // the collection who's contains() has lower O() complexity.
5375         Collection<?> contains = c2;
5376         // The collection to be iterated. If the collections' contains() impl
5377         // are of different O() complexity, the collection with slower
5378         // contains() will be used for iteration. For collections who's
5379         // contains() are of the same complexity then best performance is
5380         // achieved by iterating the smaller collection.
5381         Collection<?> iterate = c1;
5382 
5383         // Performance optimization cases. The heuristics:
5384         //   1. Generally iterate over c1.
5385         //   2. If c1 is a Set then iterate over c2.
5386         //   3. If either collection is empty then result is always true.
5387         //   4. Iterate over the smaller Collection.
5388         if (c1 instanceof Set) {
5389             // Use c1 for contains as a Set's contains() is expected to perform
5390             // better than O(N/2)
5391             iterate = c2;
5392             contains = c1;
5393         } else if (!(c2 instanceof Set)) {
5394             // Both are mere Collections. Iterate over smaller collection.
5395             // Example: If c1 contains 3 elements and c2 contains 50 elements and
5396             // assuming contains() requires ceiling(N/2) comparisons then
5397             // checking for all c1 elements in c2 would require 75 comparisons
5398             // (3 * ceiling(50/2)) vs. checking all c2 elements in c1 requiring
5399             // 100 comparisons (50 * ceiling(3/2)).
5400             int c1size = c1.size();
5401             int c2size = c2.size();
5402             if (c1size == 0 || c2size == 0) {
5403                 // At least one collection is empty. Nothing will match.
5404                 return true;
5405             }
5406 
5407             if (c1size > c2size) {
5408                 iterate = c2;
5409                 contains = c1;
5410             }
5411         }
5412 
5413         for (Object e : iterate) {
5414             if (contains.contains(e)) {
5415                // Found a common element. Collections are not disjoint.
5416                 return false;
5417             }
5418         }
5419 
5420         // No common elements were found.
5421         return true;
5422     }
5423 
5424     /**
5425      * Adds all of the specified elements to the specified collection.
5426      * Elements to be added may be specified individually or as an array.
5427      * The behavior of this convenience method is identical to that of
5428      * <tt>c.addAll(Arrays.asList(elements))</tt>, but this method is likely
5429      * to run significantly faster under most implementations.
5430      *
5431      * <p>When elements are specified individually, this method provides a
5432      * convenient way to add a few elements to an existing collection:
5433      * <pre>
5434      *     Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
5435      * </pre>
5436      *
5437      * @param  <T> the class of the elements to add and of the collection
5438      * @param c the collection into which <tt>elements</tt> are to be inserted
5439      * @param elements the elements to insert into <tt>c</tt>
5440      * @return <tt>true</tt> if the collection changed as a result of the call
5441      * @throws UnsupportedOperationException if <tt>c</tt> does not support
5442      *         the <tt>add</tt> operation
5443      * @throws NullPointerException if <tt>elements</tt> contains one or more
5444      *         null values and <tt>c</tt> does not permit null elements, or
5445      *         if <tt>c</tt> or <tt>elements</tt> are <tt>null</tt>
5446      * @throws IllegalArgumentException if some property of a value in
5447      *         <tt>elements</tt> prevents it from being added to <tt>c</tt>
5448      * @see Collection#addAll(Collection)
5449      * @since 1.5
5450      */
5451     @SafeVarargs
5452     public static <T> boolean addAll(Collection<? super T> c, T... elements) {
5453         boolean result = false;
5454         for (T element : elements)
5455             result |= c.add(element);
5456         return result;
5457     }
5458 
5459     /**
5460      * Returns a set backed by the specified map.  The resulting set displays
5461      * the same ordering, concurrency, and performance characteristics as the
5462      * backing map.  In essence, this factory method provides a {@link Set}
5463      * implementation corresponding to any {@link Map} implementation.  There
5464      * is no need to use this method on a {@link Map} implementation that
5465      * already has a corresponding {@link Set} implementation (such as {@link
5466      * HashMap} or {@link TreeMap}).
5467      *
5468      * <p>Each method invocation on the set returned by this method results in
5469      * exactly one method invocation on the backing map or its <tt>keySet</tt>
5470      * view, with one exception.  The <tt>addAll</tt> method is implemented
5471      * as a sequence of <tt>put</tt> invocations on the backing map.
5472      *
5473      * <p>The specified map must be empty at the time this method is invoked,
5474      * and should not be accessed directly after this method returns.  These
5475      * conditions are ensured if the map is created empty, passed directly
5476      * to this method, and no reference to the map is retained, as illustrated
5477      * in the following code fragment:
5478      * <pre>
5479      *    Set&lt;Object&gt; weakHashSet = Collections.newSetFromMap(
5480      *        new WeakHashMap&lt;Object, Boolean&gt;());
5481      * </pre>
5482      *
5483      * @param <E> the class of the map keys and of the objects in the
5484      *        returned set
5485      * @param map the backing map
5486      * @return the set backed by the map
5487      * @throws IllegalArgumentException if <tt>map</tt> is not empty
5488      * @since 1.6
5489      */
5490     public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
5491         return new SetFromMap<>(map);
5492     }
5493 
5494     /**
5495      * @serial include
5496      */
5497     private static class SetFromMap<E> extends AbstractSet<E>
5498         implements Set<E>, Serializable
5499     {
5500         private final Map<E, Boolean> m;  // The backing map
5501         private transient Set<E> s;       // Its keySet
5502 
5503         SetFromMap(Map<E, Boolean> map) {
5504             if (!map.isEmpty())
5505                 throw new IllegalArgumentException("Map is non-empty");
5506             m = map;
5507             s = map.keySet();
5508         }
5509 
5510         public void clear()               {        m.clear(); }
5511         public int size()                 { return m.size(); }
5512         public boolean isEmpty()          { return m.isEmpty(); }
5513         public boolean contains(Object o) { return m.containsKey(o); }
5514         public boolean remove(Object o)   { return m.remove(o) != null; }
5515         public boolean add(E e) { return m.put(e, Boolean.TRUE) == null; }
5516         public Iterator<E> iterator()     { return s.iterator(); }
5517         public Object[] toArray()         { return s.toArray(); }
5518         public <T> T[] toArray(T[] a)     { return s.toArray(a); }
5519         public String toString()          { return s.toString(); }
5520         public int hashCode()             { return s.hashCode(); }
5521         public boolean equals(Object o)   { return o == this || s.equals(o); }
5522         public boolean containsAll(Collection<?> c) {return s.containsAll(c);}
5523         public boolean removeAll(Collection<?> c)   {return s.removeAll(c);}
5524         public boolean retainAll(Collection<?> c)   {return s.retainAll(c);}
5525         // addAll is the only inherited implementation
5526 
5527         // Override default methods in Collection
5528         @Override
5529         public void forEach(Consumer<? super E> action) {
5530             s.forEach(action);
5531         }
5532         @Override
5533         public boolean removeIf(Predicate<? super E> filter) {
5534             return s.removeIf(filter);
5535         }
5536 
5537         @Override
5538         public Spliterator<E> spliterator() {return s.spliterator();}
5539         @Override
5540         public Stream<E> stream()           {return s.stream();}
5541         @Override
5542         public Stream<E> parallelStream()   {return s.parallelStream();}
5543 
5544         private static final long serialVersionUID = 2454657854757543876L;
5545 
5546         private void readObject(java.io.ObjectInputStream stream)
5547             throws IOException, ClassNotFoundException
5548         {
5549             stream.defaultReadObject();
5550             s = m.keySet();
5551         }
5552     }
5553 
5554     /**
5555      * Returns a view of a {@link Deque} as a Last-in-first-out (Lifo)
5556      * {@link Queue}. Method <tt>add</tt> is mapped to <tt>push</tt>,
5557      * <tt>remove</tt> is mapped to <tt>pop</tt> and so on. This
5558      * view can be useful when you would like to use a method
5559      * requiring a <tt>Queue</tt> but you need Lifo ordering.
5560      *
5561      * <p>Each method invocation on the queue returned by this method
5562      * results in exactly one method invocation on the backing deque, with
5563      * one exception.  The {@link Queue#addAll addAll} method is
5564      * implemented as a sequence of {@link Deque#addFirst addFirst}
5565      * invocations on the backing deque.
5566      *
5567      * @param  <T> the class of the objects in the deque
5568      * @param deque the deque
5569      * @return the queue
5570      * @since  1.6
5571      */
5572     public static <T> Queue<T> asLifoQueue(Deque<T> deque) {
5573         return new AsLIFOQueue<>(deque);
5574     }
5575 
5576     /**
5577      * @serial include
5578      */
5579     static class AsLIFOQueue<E> extends AbstractQueue<E>
5580         implements Queue<E>, Serializable {
5581         private static final long serialVersionUID = 1802017725587941708L;
5582         private final Deque<E> q;
5583         AsLIFOQueue(Deque<E> q)           { this.q = q; }
5584         public boolean add(E e)           { q.addFirst(e); return true; }
5585         public boolean offer(E e)         { return q.offerFirst(e); }
5586         public E poll()                   { return q.pollFirst(); }
5587         public E remove()                 { return q.removeFirst(); }
5588         public E peek()                   { return q.peekFirst(); }
5589         public E element()                { return q.getFirst(); }
5590         public void clear()               {        q.clear(); }
5591         public int size()                 { return q.size(); }
5592         public boolean isEmpty()          { return q.isEmpty(); }
5593         public boolean contains(Object o) { return q.contains(o); }
5594         public boolean remove(Object o)   { return q.remove(o); }
5595         public Iterator<E> iterator()     { return q.iterator(); }
5596         public Object[] toArray()         { return q.toArray(); }
5597         public <T> T[] toArray(T[] a)     { return q.toArray(a); }
5598         public String toString()          { return q.toString(); }
5599         public boolean containsAll(Collection<?> c) {return q.containsAll(c);}
5600         public boolean removeAll(Collection<?> c)   {return q.removeAll(c);}
5601         public boolean retainAll(Collection<?> c)   {return q.retainAll(c);}
5602         // We use inherited addAll; forwarding addAll would be wrong
5603 
5604         // Override default methods in Collection
5605         @Override
5606         public void forEach(Consumer<? super E> action) {q.forEach(action);}
5607         @Override
5608         public boolean removeIf(Predicate<? super E> filter) {
5609             return q.removeIf(filter);
5610         }
5611         @Override
5612         public Spliterator<E> spliterator() {return q.spliterator();}
5613         @Override
5614         public Stream<E> stream()           {return q.stream();}
5615         @Override
5616         public Stream<E> parallelStream()   {return q.parallelStream();}
5617     }
5618 }
5619