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revision 1.17, Fri Sep 12 15:38:26 2003 UTC revision 1.28, Mon May 2 17:34:02 2005 UTC
# Line 1  Line 1 
1  /*  /*
2   * Written by Doug Lea with assistance from members of JCP JSR-166   * Written by Doug Lea with assistance from members of JCP JSR-166
3   * Expert Group and released to the public domain. Use, modify, and   * Expert Group and released to the public domain, as explained at
4   * redistribute this code in any way without acknowledgement.   * http://creativecommons.org/licenses/publicdomain
5   */   */
6    
7  package java.util;  package java.util;
8    
9  /**  /**
10   * A collection designed for holding elements prior to processing.   * A collection designed for holding elements prior to processing.
11   * Besides basic {@link java.util.Collection Collection} operations, queues provide   * Besides basic {@link java.util.Collection Collection} operations,
12   * additional insertion, extraction, and inspection operations.   * queues provide additional insertion, extraction, and inspection
13     * operations.  Each of these methods exists in two forms: one throws
14     * an exception if the operation fails, the other returns a special
15     * value (either <tt>null</tt> or <tt>false</tt>, depending on the
16     * operation).  The latter form of the insert operation is designed
17     * specifically for use with capacity-restricted <tt>Queue</tt>
18     * implementations; in most implementations, insert operations cannot
19     * fail.
20     *
21     * <p>
22     * <table BORDER CELLPADDING=3 CELLSPACING=1>
23     *  <tr>
24     *    <td></td>
25     *    <td ALIGN=CENTER><em>Throws exception</em></td>
26     *    <td ALIGN=CENTER><em>Returns special value</em></td>
27     *  </tr>
28     *  <tr>
29     *    <td><b>Insert</b></td>
30     *    <td>{@link #add add(e)}</td>
31     *    <td>{@link #offer offer(e)}</td>
32     *  </tr>
33     *  <tr>
34     *    <td><b>Remove</b></td>
35     *    <td>{@link #remove remove()}</td>
36     *    <td>{@link #poll poll()}</td>
37     *  </tr>
38     *  <tr>
39     *    <td><b>Examine</b></td>
40     *    <td>{@link #element element()}</td>
41     *    <td>{@link #peek peek()}</td>
42     *  </tr>
43     * </table>
44   *   *
45   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
46   * FIFO (first-in-first-out) manner.  Among the exceptions are   * FIFO (first-in-first-out) manner.  Among the exceptions are
# Line 79  Line 110 
110   * @see java.util.concurrent.PriorityBlockingQueue   * @see java.util.concurrent.PriorityBlockingQueue
111   * @since 1.5   * @since 1.5
112   * @author Doug Lea   * @author Doug Lea
113     * @param <E> the type of elements held in this collection
114   */   */
115  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
116    
117      /**      /**
118       * Inserts the specified element to this queue, if possible.  When       * Inserts the specified element into this queue, if possible.  When
119       * using queues that may impose insertion restrictions (for       * using queues that may impose insertion restrictions (for
120       * example capacity bounds), method <tt>offer</tt> is generally       * example capacity bounds), method <tt>offer</tt> is generally
121       * preferable to method {@link Collection#add}, which can fail to       * preferable to method {@link Collection#add}, which can fail to
122       * insert an element only by throwing an exception.       * insert an element only by throwing an exception.
123       *       *
124       * @param o the element to insert.       * @param e the element to insert.
125       * @return <tt>true</tt> if it was possible to add the element to       * @return <tt>true</tt> if it was possible to add the element to
126       * this queue, else <tt>false</tt>       * this queue, else <tt>false</tt>
127       */       */
128      boolean offer(E o);      boolean offer(E e);
129    
130      /**      /**
131       * Retrieves and removes the head of this queue, if it is available.       * Retrieves and removes the head of this queue, or <tt>null</tt>
132         * if this queue is empty.
133       *       *
134       * @return the head of this queue, or <tt>null</tt> if this       * @return the head of this queue, or <tt>null</tt> if this
135       *         queue is empty.       *         queue is empty.
# Line 104  Line 137 
137      E poll();      E poll();
138    
139      /**      /**
140       * Retrieves and removes the head of this queue.       * Retrieves and removes the head of this queue.  This method
141       * This method differs       * differs from the {@link #poll} method only in that it throws an
142       * from the <tt>poll</tt> method in that it throws an exception if this       * exception if this queue is empty.
      * queue is empty.  
143       *       *
144       * @return the head of this queue.       * @return the head of this queue.
145       * @throws NoSuchElementException if this queue is empty.       * @throws NoSuchElementException if this queue is empty.
# Line 115  Line 147 
147      E remove();      E remove();
148    
149      /**      /**
150       * Retrieves, but does not remove, the head of this queue.       * Retrieves, but does not remove, the head of this queue,
151       * This method differs from the <tt>poll</tt>       * returning <tt>null</tt> if this queue is empty.
      * method only in that this method does not remove the head element from  
      * this queue.  
152       *       *
153       * @return the head of this queue, or <tt>null</tt> if this queue is empty.       * @return the head of this queue, or <tt>null</tt> if this queue
154         * is empty.
155       */       */
156      E peek();      E peek();
157    
158      /**      /**
159       * Retrieves, but does not remove, the head of this queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
160       * differs from the <tt>peek</tt> method only in that it throws an       * differs from the {@link #peek} method only in that it throws an
161       * exception if this queue is empty.       * exception if this queue is empty.
162       *       *
163       * @return the head of this queue.       * @return the head of this queue.

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