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3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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11   * This code is distributed in the hope that it will be useful, but WITHOUT
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13   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14   * version 2 for more details (a copy is included in the LICENSE file that
15   * accompanied this code).
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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       */
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       */
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       */
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      */
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      */
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      */
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      */
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      */
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      */
194     void add(E e);
195 }