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25 
26 package java.util;
27 
28 /**
29  * An iterator for lists that allows the programmer
30  * to traverse the list in either direction, modify
31  * the list during iteration, and obtain the iterator's
32  * current position in the list. A {@code ListIterator}
33  * has no current element; its <I>cursor position</I> always
34  * lies between the element that would be returned by a call
35  * to {@code previous()} and the element that would be
36  * returned by a call to {@code next()}.
37  * An iterator for a list of length {@code n} has {@code n+1} possible
38  * cursor positions, as illustrated by the carets ({@code ^}) below:
39  * <PRE>
40  *                      Element(0)   Element(1)   Element(2)   ... Element(n-1)
41  * cursor positions:  ^            ^            ^            ^                  ^
42  * </PRE>
43  * Note that the {@link #remove} and {@link #set(Object)} methods are
44  * <i>not</i> defined in terms of the cursor position;  they are defined to
45  * operate on the last element returned by a call to {@link #next} or
46  * {@link #previous()}.
47  *
48  * <p>This interface is a member of the
49  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
50  * Java Collections Framework</a>.
51  *
52  * @author  Josh Bloch
53  * @see Collection
54  * @see List
55  * @see Iterator
56  * @see Enumeration
57  * @see List#listIterator()
58  * @since   1.2
59  */
60 public interface ListIterator<E> extends Iterator<E> {
61     // Query Operations
62 
63     /**
64      * Returns {@code true} if this list iterator has more elements when
65      * traversing the list in the forward direction. (In other words,
66      * returns {@code true} if {@link #next} would return an element rather
67      * than throwing an exception.)
68      *
69      * @return {@code true} if the list iterator has more elements when
70      *         traversing the list in the forward direction
71      */
hasNext()72     boolean hasNext();
73 
74     /**
75      * Returns the next element in the list and advances the cursor position.
76      * This method may be called repeatedly to iterate through the list,
77      * or intermixed with calls to {@link #previous} to go back and forth.
78      * (Note that alternating calls to {@code next} and {@code previous}
79      * will return the same element repeatedly.)
80      *
81      * @return the next element in the list
82      * @throws NoSuchElementException if the iteration has no next element
83      */
next()84     E next();
85 
86     /**
87      * Returns {@code true} if this list iterator has more elements when
88      * traversing the list in the reverse direction.  (In other words,
89      * returns {@code true} if {@link #previous} would return an element
90      * rather than throwing an exception.)
91      *
92      * @return {@code true} if the list iterator has more elements when
93      *         traversing the list in the reverse direction
94      */
hasPrevious()95     boolean hasPrevious();
96 
97     /**
98      * Returns the previous element in the list and moves the cursor
99      * position backwards.  This method may be called repeatedly to
100      * iterate through the list backwards, or intermixed with calls to
101      * {@link #next} to go back and forth.  (Note that alternating calls
102      * to {@code next} and {@code previous} will return the same
103      * element repeatedly.)
104      *
105      * @return the previous element in the list
106      * @throws NoSuchElementException if the iteration has no previous
107      *         element
108      */
previous()109     E previous();
110 
111     /**
112      * Returns the index of the element that would be returned by a
113      * subsequent call to {@link #next}. (Returns list size if the list
114      * iterator is at the end of the list.)
115      *
116      * @return the index of the element that would be returned by a
117      *         subsequent call to {@code next}, or list size if the list
118      *         iterator is at the end of the list
119      */
nextIndex()120     int nextIndex();
121 
122     /**
123      * Returns the index of the element that would be returned by a
124      * subsequent call to {@link #previous}. (Returns -1 if the list
125      * iterator is at the beginning of the list.)
126      *
127      * @return the index of the element that would be returned by a
128      *         subsequent call to {@code previous}, or -1 if the list
129      *         iterator is at the beginning of the list
130      */
previousIndex()131     int previousIndex();
132 
133 
134     // Modification Operations
135 
136     /**
137      * Removes from the list the last element that was returned by {@link
138      * #next} or {@link #previous} (optional operation).  This call can
139      * only be made once per call to {@code next} or {@code previous}.
140      * It can be made only if {@link #add} has not been
141      * called after the last call to {@code next} or {@code previous}.
142      *
143      * @throws UnsupportedOperationException if the {@code remove}
144      *         operation is not supported by this list iterator
145      * @throws IllegalStateException if neither {@code next} nor
146      *         {@code previous} have been called, or {@code remove} or
147      *         {@code add} have been called after the last call to
148      *         {@code next} or {@code previous}
149      */
remove()150     void remove();
151 
152     /**
153      * Replaces the last element returned by {@link #next} or
154      * {@link #previous} with the specified element (optional operation).
155      * This call can be made only if neither {@link #remove} nor {@link
156      * #add} have been called after the last call to {@code next} or
157      * {@code previous}.
158      *
159      * @param e the element with which to replace the last element returned by
160      *          {@code next} or {@code previous}
161      * @throws UnsupportedOperationException if the {@code set} operation
162      *         is not supported by this list iterator
163      * @throws ClassCastException if the class of the specified element
164      *         prevents it from being added to this list
165      * @throws IllegalArgumentException if some aspect of the specified
166      *         element prevents it from being added to this list
167      * @throws IllegalStateException if neither {@code next} nor
168      *         {@code previous} have been called, or {@code remove} or
169      *         {@code add} have been called after the last call to
170      *         {@code next} or {@code previous}
171      */
set(E e)172     void set(E e);
173 
174     /**
175      * Inserts the specified element into the list (optional operation).
176      * The element is inserted immediately before the element that
177      * would be returned by {@link #next}, if any, and after the element
178      * that would be returned by {@link #previous}, if any.  (If the
179      * list contains no elements, the new element becomes the sole element
180      * on the list.)  The new element is inserted before the implicit
181      * cursor: a subsequent call to {@code next} would be unaffected, and a
182      * subsequent call to {@code previous} would return the new element.
183      * (This call increases by one the value that would be returned by a
184      * call to {@code nextIndex} or {@code previousIndex}.)
185      *
186      * @param e the element to insert
187      * @throws UnsupportedOperationException if the {@code add} method is
188      *         not supported by this list iterator
189      * @throws ClassCastException if the class of the specified element
190      *         prevents it from being added to this list
191      * @throws IllegalArgumentException if some aspect of this element
192      *         prevents it from being added to this list
193      */
add(E e)194     void add(E e);
195 }
196