1 /* LinkedHashMap.java -- a class providing hashtable data structure, 2 mapping Object --> Object, with linked list traversal 3 Copyright (C) 2001, 2002, 2005 Free Software Foundation, Inc. 4 5 This file is part of GNU Classpath. 6 7 GNU Classpath is free software; you can redistribute it and/or modify 8 it under the terms of the GNU General Public License as published by 9 the Free Software Foundation; either version 2, or (at your option) 10 any later version. 11 12 GNU Classpath is distributed in the hope that it will be useful, but 13 WITHOUT ANY WARRANTY; without even the implied warranty of 14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 15 General Public License for more details. 16 17 You should have received a copy of the GNU General Public License 18 along with GNU Classpath; see the file COPYING. If not, write to the 19 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 20 02110-1301 USA. 21 22 Linking this library statically or dynamically with other modules is 23 making a combined work based on this library. Thus, the terms and 24 conditions of the GNU General Public License cover the whole 25 combination. 26 27 As a special exception, the copyright holders of this library give you 28 permission to link this library with independent modules to produce an 29 executable, regardless of the license terms of these independent 30 modules, and to copy and distribute the resulting executable under 31 terms of your choice, provided that you also meet, for each linked 32 independent module, the terms and conditions of the license of that 33 module. An independent module is a module which is not derived from 34 or based on this library. If you modify this library, you may extend 35 this exception to your version of the library, but you are not 36 obligated to do so. If you do not wish to do so, delete this 37 exception statement from your version. */ 38 39 40 package java.util; 41 42 /** 43 * This class provides a hashtable-backed implementation of the 44 * Map interface, with predictable traversal order. 45 * <p> 46 * 47 * It uses a hash-bucket approach; that is, hash collisions are handled 48 * by linking the new node off of the pre-existing node (or list of 49 * nodes). In this manner, techniques such as linear probing (which 50 * can cause primary clustering) and rehashing (which does not fit very 51 * well with Java's method of precomputing hash codes) are avoided. In 52 * addition, this maintains a doubly-linked list which tracks either 53 * insertion or access order. 54 * <p> 55 * 56 * In insertion order, calling <code>put</code> adds the key to the end of 57 * traversal, unless the key was already in the map; changing traversal order 58 * requires removing and reinserting a key. On the other hand, in access 59 * order, all calls to <code>put</code> and <code>get</code> cause the 60 * accessed key to move to the end of the traversal list. Note that any 61 * accesses to the map's contents via its collection views and iterators do 62 * not affect the map's traversal order, since the collection views do not 63 * call <code>put</code> or <code>get</code>. 64 * <p> 65 * 66 * One of the nice features of tracking insertion order is that you can 67 * copy a hashtable, and regardless of the implementation of the original, 68 * produce the same results when iterating over the copy. This is possible 69 * without needing the overhead of <code>TreeMap</code>. 70 * <p> 71 * 72 * When using this {@link #LinkedHashMap(int, float, boolean) constructor}, 73 * you can build an access-order mapping. This can be used to implement LRU 74 * caches, for example. By overriding {@link #removeEldestEntry(Map.Entry)}, 75 * you can also control the removal of the oldest entry, and thereby do 76 * things like keep the map at a fixed size. 77 * <p> 78 * 79 * Under ideal circumstances (no collisions), LinkedHashMap offers O(1) 80 * performance on most operations (<code>containsValue()</code> is, 81 * of course, O(n)). In the worst case (all keys map to the same 82 * hash code -- very unlikely), most operations are O(n). Traversal is 83 * faster than in HashMap (proportional to the map size, and not the space 84 * allocated for the map), but other operations may be slower because of the 85 * overhead of the maintaining the traversal order list. 86 * <p> 87 * 88 * LinkedHashMap accepts the null key and null values. It is not 89 * synchronized, so if you need multi-threaded access, consider using:<br> 90 * <code>Map m = Collections.synchronizedMap(new LinkedHashMap(...));</code> 91 * <p> 92 * 93 * The iterators are <i>fail-fast</i>, meaning that any structural 94 * modification, except for <code>remove()</code> called on the iterator 95 * itself, cause the iterator to throw a 96 * {@link ConcurrentModificationException} rather than exhibit 97 * non-deterministic behavior. 98 * 99 * @author Eric Blake (ebb9@email.byu.edu) 100 * @author Tom Tromey (tromey@redhat.com) 101 * @author Andrew John Hughes (gnu_andrew@member.fsf.org) 102 * @see Object#hashCode() 103 * @see Collection 104 * @see Map 105 * @see HashMap 106 * @see TreeMap 107 * @see Hashtable 108 * @since 1.4 109 * @status updated to 1.4 110 */ 111 public class LinkedHashMap<K,V> extends HashMap<K,V> 112 { 113 /** 114 * Compatible with JDK 1.4. 115 */ 116 private static final long serialVersionUID = 3801124242820219131L; 117 118 /** 119 * The oldest Entry to begin iteration at. 120 */ 121 transient LinkedHashEntry root; 122 123 /** 124 * The iteration order of this linked hash map: <code>true</code> for 125 * access-order, <code>false</code> for insertion-order. 126 * 127 * @serial true for access order traversal 128 */ 129 final boolean accessOrder; 130 131 /** 132 * Class to represent an entry in the hash table. Holds a single key-value 133 * pair and the doubly-linked insertion order list. 134 */ 135 class LinkedHashEntry<K,V> extends HashEntry<K,V> 136 { 137 /** 138 * The predecessor in the iteration list. If this entry is the root 139 * (eldest), pred points to the newest entry. 140 */ 141 LinkedHashEntry<K,V> pred; 142 143 /** The successor in the iteration list, null if this is the newest. */ 144 LinkedHashEntry<K,V> succ; 145 146 /** 147 * Simple constructor. 148 * 149 * @param key the key 150 * @param value the value 151 */ LinkedHashEntry(K key, V value)152 LinkedHashEntry(K key, V value) 153 { 154 super(key, value); 155 if (root == null) 156 { 157 root = this; 158 pred = this; 159 } 160 else 161 { 162 pred = root.pred; 163 pred.succ = this; 164 root.pred = this; 165 } 166 } 167 168 /** 169 * Called when this entry is accessed via put or get. This version does 170 * the necessary bookkeeping to keep the doubly-linked list in order, 171 * after moving this element to the newest position in access order. 172 */ access()173 void access() 174 { 175 if (accessOrder && succ != null) 176 { 177 modCount++; 178 if (this == root) 179 { 180 root = succ; 181 pred.succ = this; 182 succ = null; 183 } 184 else 185 { 186 pred.succ = succ; 187 succ.pred = pred; 188 succ = null; 189 pred = root.pred; 190 pred.succ = this; 191 root.pred = this; 192 } 193 } 194 } 195 196 /** 197 * Called when this entry is removed from the map. This version does 198 * the necessary bookkeeping to keep the doubly-linked list in order. 199 * 200 * @return the value of this key as it is removed 201 */ cleanup()202 V cleanup() 203 { 204 if (this == root) 205 { 206 root = succ; 207 if (succ != null) 208 succ.pred = pred; 209 } 210 else if (succ == null) 211 { 212 pred.succ = null; 213 root.pred = pred; 214 } 215 else 216 { 217 pred.succ = succ; 218 succ.pred = pred; 219 } 220 return value; 221 } 222 } // class LinkedHashEntry 223 224 /** 225 * Construct a new insertion-ordered LinkedHashMap with the default 226 * capacity (11) and the default load factor (0.75). 227 */ LinkedHashMap()228 public LinkedHashMap() 229 { 230 super(); 231 accessOrder = false; 232 } 233 234 /** 235 * Construct a new insertion-ordered LinkedHashMap from the given Map, 236 * with initial capacity the greater of the size of <code>m</code> or 237 * the default of 11. 238 * <p> 239 * 240 * Every element in Map m will be put into this new HashMap, in the 241 * order of m's iterator. 242 * 243 * @param m a Map whose key / value pairs will be put into 244 * the new HashMap. <b>NOTE: key / value pairs 245 * are not cloned in this constructor.</b> 246 * @throws NullPointerException if m is null 247 */ LinkedHashMap(Map<? extends K, ? extends V> m)248 public LinkedHashMap(Map<? extends K, ? extends V> m) 249 { 250 super(m); 251 accessOrder = false; 252 } 253 254 /** 255 * Construct a new insertion-ordered LinkedHashMap with a specific 256 * inital capacity and default load factor of 0.75. 257 * 258 * @param initialCapacity the initial capacity of this HashMap (>= 0) 259 * @throws IllegalArgumentException if (initialCapacity < 0) 260 */ LinkedHashMap(int initialCapacity)261 public LinkedHashMap(int initialCapacity) 262 { 263 super(initialCapacity); 264 accessOrder = false; 265 } 266 267 /** 268 * Construct a new insertion-orderd LinkedHashMap with a specific 269 * inital capacity and load factor. 270 * 271 * @param initialCapacity the initial capacity (>= 0) 272 * @param loadFactor the load factor (> 0, not NaN) 273 * @throws IllegalArgumentException if (initialCapacity < 0) || 274 * ! (loadFactor > 0.0) 275 */ LinkedHashMap(int initialCapacity, float loadFactor)276 public LinkedHashMap(int initialCapacity, float loadFactor) 277 { 278 super(initialCapacity, loadFactor); 279 accessOrder = false; 280 } 281 282 /** 283 * Construct a new LinkedHashMap with a specific inital capacity, load 284 * factor, and ordering mode. 285 * 286 * @param initialCapacity the initial capacity (>=0) 287 * @param loadFactor the load factor (>0, not NaN) 288 * @param accessOrder true for access-order, false for insertion-order 289 * @throws IllegalArgumentException if (initialCapacity < 0) || 290 * ! (loadFactor > 0.0) 291 */ LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)292 public LinkedHashMap(int initialCapacity, float loadFactor, 293 boolean accessOrder) 294 { 295 super(initialCapacity, loadFactor); 296 this.accessOrder = accessOrder; 297 } 298 299 /** 300 * Clears the Map so it has no keys. This is O(1). 301 */ clear()302 public void clear() 303 { 304 super.clear(); 305 root = null; 306 } 307 308 /** 309 * Returns <code>true</code> if this HashMap contains a value 310 * <code>o</code>, such that <code>o.equals(value)</code>. 311 * 312 * @param value the value to search for in this HashMap 313 * @return <code>true</code> if at least one key maps to the value 314 */ containsValue(Object value)315 public boolean containsValue(Object value) 316 { 317 LinkedHashEntry e = root; 318 while (e != null) 319 { 320 if (equals(value, e.value)) 321 return true; 322 e = e.succ; 323 } 324 return false; 325 } 326 327 /** 328 * Return the value in this Map associated with the supplied key, 329 * or <code>null</code> if the key maps to nothing. If this is an 330 * access-ordered Map and the key is found, this performs structural 331 * modification, moving the key to the newest end of the list. NOTE: 332 * Since the value could also be null, you must use containsKey to 333 * see if this key actually maps to something. 334 * 335 * @param key the key for which to fetch an associated value 336 * @return what the key maps to, if present 337 * @see #put(Object, Object) 338 * @see #containsKey(Object) 339 */ get(Object key)340 public V get(Object key) 341 { 342 int idx = hash(key); 343 HashEntry<K,V> e = buckets[idx]; 344 while (e != null) 345 { 346 if (equals(key, e.key)) 347 { 348 e.access(); 349 return e.value; 350 } 351 e = e.next; 352 } 353 return null; 354 } 355 356 /** 357 * Returns <code>true</code> if this map should remove the eldest entry. 358 * This method is invoked by all calls to <code>put</code> and 359 * <code>putAll</code> which place a new entry in the map, providing 360 * the implementer an opportunity to remove the eldest entry any time 361 * a new one is added. This can be used to save memory usage of the 362 * hashtable, as well as emulating a cache, by deleting stale entries. 363 * <p> 364 * 365 * For example, to keep the Map limited to 100 entries, override as follows: 366 * <pre> 367 * private static final int MAX_ENTRIES = 100; 368 * protected boolean removeEldestEntry(Map.Entry eldest) 369 * { 370 * return size() > MAX_ENTRIES; 371 * } 372 * </pre><p> 373 * 374 * Typically, this method does not modify the map, but just uses the 375 * return value as an indication to <code>put</code> whether to proceed. 376 * However, if you override it to modify the map, you must return false 377 * (indicating that <code>put</code> should leave the modified map alone), 378 * or you face unspecified behavior. Remember that in access-order mode, 379 * even calling <code>get</code> is a structural modification, but using 380 * the collections views (such as <code>keySet</code>) is not. 381 * <p> 382 * 383 * This method is called after the eldest entry has been inserted, so 384 * if <code>put</code> was called on a previously empty map, the eldest 385 * entry is the one you just put in! The default implementation just 386 * returns <code>false</code>, so that this map always behaves like 387 * a normal one with unbounded growth. 388 * 389 * @param eldest the eldest element which would be removed if this 390 * returns true. For an access-order map, this is the least 391 * recently accessed; for an insertion-order map, this is the 392 * earliest element inserted. 393 * @return true if <code>eldest</code> should be removed 394 */ removeEldestEntry(Map.Entry<K,V> eldest)395 protected boolean removeEldestEntry(Map.Entry<K,V> eldest) 396 { 397 return false; 398 } 399 400 /** 401 * Helper method called by <code>put</code>, which creates and adds a 402 * new Entry, followed by performing bookkeeping (like removeEldestEntry). 403 * 404 * @param key the key of the new Entry 405 * @param value the value 406 * @param idx the index in buckets where the new Entry belongs 407 * @param callRemove whether to call the removeEldestEntry method 408 * @see #put(Object, Object) 409 * @see #removeEldestEntry(Map.Entry) 410 * @see LinkedHashEntry#LinkedHashEntry(Object, Object) 411 */ addEntry(K key, V value, int idx, boolean callRemove)412 void addEntry(K key, V value, int idx, boolean callRemove) 413 { 414 LinkedHashEntry e = new LinkedHashEntry(key, value); 415 e.next = buckets[idx]; 416 buckets[idx] = e; 417 if (callRemove && removeEldestEntry(root)) 418 remove(root.key); 419 } 420 421 /** 422 * Helper method, called by clone() to reset the doubly-linked list. 423 * 424 * @param m the map to add entries from 425 * @see #clone() 426 */ putAllInternal(Map m)427 void putAllInternal(Map m) 428 { 429 root = null; 430 super.putAllInternal(m); 431 } 432 433 /** 434 * Generates a parameterized iterator. This allows traversal to follow 435 * the doubly-linked list instead of the random bin order of HashMap. 436 * 437 * @param type {@link #KEYS}, {@link #VALUES}, or {@link #ENTRIES} 438 * @return the appropriate iterator 439 */ iterator(final int type)440 Iterator iterator(final int type) 441 { 442 return new Iterator() 443 { 444 /** The current Entry. */ 445 LinkedHashEntry current = root; 446 447 /** The previous Entry returned by next(). */ 448 LinkedHashEntry last; 449 450 /** The number of known modifications to the backing Map. */ 451 int knownMod = modCount; 452 453 /** 454 * Returns true if the Iterator has more elements. 455 * 456 * @return true if there are more elements 457 */ 458 public boolean hasNext() 459 { 460 return current != null; 461 } 462 463 /** 464 * Returns the next element in the Iterator's sequential view. 465 * 466 * @return the next element 467 * @throws ConcurrentModificationException if the HashMap was modified 468 * @throws NoSuchElementException if there is none 469 */ 470 public Object next() 471 { 472 if (knownMod != modCount) 473 throw new ConcurrentModificationException(); 474 if (current == null) 475 throw new NoSuchElementException(); 476 last = current; 477 current = current.succ; 478 return type == VALUES ? last.value : type == KEYS ? last.key : last; 479 } 480 481 /** 482 * Removes from the backing HashMap the last element which was fetched 483 * with the <code>next()</code> method. 484 * 485 * @throws ConcurrentModificationException if the HashMap was modified 486 * @throws IllegalStateException if called when there is no last element 487 */ 488 public void remove() 489 { 490 if (knownMod != modCount) 491 throw new ConcurrentModificationException(); 492 if (last == null) 493 throw new IllegalStateException(); 494 LinkedHashMap.this.remove(last.key); 495 last = null; 496 knownMod++; 497 } 498 }; 499 } 500 } // class LinkedHashMap 501