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revision 1.19, Mon Sep 15 12:02:23 2003 UTC revision 1.37, Sun Nov 18 19:33:08 2012 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/publicdomain/zero/1.0/
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 66  Line 97 
97   *   *
98   *   *
99   * <p>This interface is a member of the   * <p>This interface is a member of the
100   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
101   * Java Collections Framework</a>.   * Java Collections Framework</a>.
102   *   *
103   * @see java.util.Collection   * @see java.util.Collection
# 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         * Inserts the specified element into this queue if it is possible to do so
118         * immediately without violating capacity restrictions, returning
119         * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
120         * if no space is currently available.
121         *
122         * @param e the element to add
123         * @return <tt>true</tt> (as specified by {@link Collection#add})
124         * @throws IllegalStateException if the element cannot be added at this
125         *         time due to capacity restrictions
126         * @throws ClassCastException if the class of the specified element
127         *         prevents it from being added to this queue
128         * @throws NullPointerException if the specified element is null and
129         *         this queue does not permit null elements
130         * @throws IllegalArgumentException if some property of this element
131         *         prevents it from being added to this queue
132         */
133        boolean add(E e);
134    
135        /**
136         * Inserts the specified element into this queue if it is possible to do
137         * so immediately without violating capacity restrictions.
138         * When using a capacity-restricted queue, this method is generally
139         * preferable to {@link #add}, which can fail to insert an element only
140         * by throwing an exception.
141         *
142         * @param e the element to add
143         * @return <tt>true</tt> if the element was added to this queue, else
144         *         <tt>false</tt>
145         * @throws ClassCastException if the class of the specified element
146         *         prevents it from being added to this queue
147         * @throws NullPointerException if the specified element is null and
148         *         this queue does not permit null elements
149         * @throws IllegalArgumentException if some property of this element
150         *         prevents it from being added to this queue
151         */
152        boolean offer(E e);
153    
154      /**      /**
155       * Inserts the specified element into this queue, if possible.  When       * Retrieves and removes the head of this queue.  This method differs
156       * using queues that may impose insertion restrictions (for       * from {@link #poll poll} only in that it throws an exception if this
157       * example capacity bounds), method <tt>offer</tt> is generally       * queue is empty.
      * preferable to method {@link Collection#add}, which can fail to  
      * insert an element only by throwing an exception.  
158       *       *
159       * @param o the element to insert.       * @return the head of this queue
160       * @return <tt>true</tt> if it was possible to add the element to       * @throws NoSuchElementException if this queue is empty
      * this queue, else <tt>false</tt>  
161       */       */
162      boolean offer(E o);      E remove();
163    
164      /**      /**
165       * Retrieves and removes the head of this queue, or <tt>null</tt>       * Retrieves and removes the head of this queue,
166       * if this queue is empty.       * or returns <tt>null</tt> if this queue is empty.
167       *       *
168       * @return the head of this queue, or <tt>null</tt> if this       * @return the head of this queue, or <tt>null</tt> if this queue is empty
      *         queue is empty.  
169       */       */
170      E poll();      E poll();
171    
172      /**      /**
173       * Retrieves and removes the head of this queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
174       * differs from the <tt>poll</tt> method in that it throws an       * differs from {@link #peek peek} only in that it throws an exception
175       * exception if this queue is empty.       * if this queue is empty.
176       *       *
177       * @return the head of this queue.       * @return the head of this queue
178       * @throws NoSuchElementException if this queue is empty.       * @throws NoSuchElementException if this queue is empty
179       */       */
180      E remove();      E element();
181    
182      /**      /**
183       * Retrieves, but does not remove, the head of this queue,       * Retrieves, but does not remove, the head of this queue,
184       * returning <tt>null</tt> if this queue is empty.       * or returns <tt>null</tt> if this queue is empty.
185       *       *
186       * @return the head of this queue, or <tt>null</tt> if this queue       * @return the head of this queue, or <tt>null</tt> if this queue is empty
      * is empty.  
187       */       */
188      E peek();      E peek();
   
     /**  
      * Retrieves, but does not remove, the head of this queue.  This method  
      * differs from the <tt>peek</tt> method only in that it throws an  
      * exception if this queue is empty.  
      *  
      * @return the head of this queue.  
      * @throws NoSuchElementException if this queue is empty.  
      */  
     E element();  
189  }  }

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