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revision 1.7, Tue Jun 24 14:34:30 2003 UTC revision 1.23, Sun Nov 21 01:40:39 2004 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 Collection} operations, queues provide   * Besides basic {@link java.util.Collection Collection} operations, queues provide
12   * additional insertion, extraction, and inspection operations.   * additional insertion, extraction, and inspection operations. The names
13  0 *   * of the operations vary with their policies:
14     *
15     *<table BORDER CELLPADDING=3 CELLSPACING=1>
16     *  <tr>
17     *    <td><em>Operation</em></td>
18     *    <td ALIGN=CENTER><em>Attempt</em></td>
19     *    <td ALIGN=CENTER><em>Throw</em></td>
20     *  </tr>
21     *  <tr>
22     *    <td><em>insert</em></td>
23     *    <td>offer(x)</td>
24     *    <td>add(x)</td>
25     *  </tr>
26     *  <tr>
27     *    <td><em>extract</em></td>
28     *    <td>poll()</td>
29     *    <td>remove()</td>
30     *  </tr>
31     *  <tr>
32     *    <td><em>inspect</em></td>
33     *    <td>peek()</td>
34     *    <td>element()</td>
35     *  </tr>
36     *</table>
37     *
38   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
39   * FIFO (first-in-first-out) manner.  Among the exceptions are   * FIFO (first-in-first-out) manner.  Among the exceptions are
40   * priority queues, which order elements according to a supplied   * priority queues, which order elements according to a supplied
41   * comparator, or the elements' natural ordering.  Every Queue   * comparator, or the elements' natural ordering, and LIFO queues (or
42   * implementation must specify its ordering guarantees.   * stacks) which order the elements LIFO (last-in-first-out).
43   *   * Whatever the ordering used, the <em>head</em> of the queue is that
44   * <p>The {@link #offer(E)} method adds an element if possible, otherwise   * element which would be removed by a call to {@link #remove() } or
45   * returning <tt>false</tt>.  This differs from the {@link   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
46   * Collections#add(Object)} method, which throws an unchecked exception upon   * the <em> tail</em> of the queue. Other kinds of queues may use
47   * failure. It is designed for use in collections in which failure to   * different placement rules.  Every <tt>Queue</tt> implementation
48   * add is a normal, rather than exceptional occurrence, for example,   * must specify its ordering properties.
49   * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.   *
50     * <p>The {@link #offer offer} method inserts an element if possible,
51     * otherwise returning <tt>false</tt>.  This differs from the {@link
52     * java.util.Collection#add Collection.add} method, which can fail to
53     * add an element only by throwing an unchecked exception.  The
54     * <tt>offer</tt> method is designed for use when failure is a normal,
55     * rather than exceptional occurrence, for example, in fixed-capacity
56     * (or &quot;bounded&quot;) queues.
57   *   *
58   * <p>The {@link #remove()} and {@link #poll()} methods remove and   * <p>The {@link #remove()} and {@link #poll()} methods remove and
59   * return an element in accord with the implementation's ordering   * return the head of the queue.
60   * policy.  Exactly which element is removed from the queue is a   * Exactly which element is removed from the queue is a
61   * function of the queue's ordering policy, which differs from   * function of the queue's ordering policy, which differs from
62   * implementation to implementation.  Possible orderings include (but   * implementation to implementation. The <tt>remove()</tt> and
  * are not limited to) first-in-first-out (FIFO), last-in-first-out  
  * (LIFO), element priority, and arbitrary.  The <tt>remove()</tt> and  
63   * <tt>poll()</tt> methods differ only in their behavior when the   * <tt>poll()</tt> methods differ only in their behavior when the
64   * queue is empty: the <tt>remove()</tt> method throws an exception,   * queue is empty: the <tt>remove()</tt> method throws an exception,
65   * while the <tt>poll()</tt> method returns <tt>null</tt>.   * while the <tt>poll()</tt> method returns <tt>null</tt>.
66   *   *
67   * <p>The {@link #element()} and {@link #peek()} methods return but do   * <p>The {@link #element()} and {@link #peek()} methods return, but do
68   * not delete the element that would be obtained by a call to   * not remove, the head of the queue.
  * the <tt>remove</tt> and <tt>poll</tt> methods respectively.  
69   *   *
70   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
71   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
# Line 54  Line 81 
81   * used as a special return value by the <tt>poll</tt> method to   * used as a special return value by the <tt>poll</tt> method to
82   * indicate that the queue contains no elements.   * indicate that the queue contains no elements.
83   *   *
84     * <p><tt>Queue</tt> implementations generally do not define
85     * element-based versions of methods <tt>equals</tt> and
86     * <tt>hashCode</tt> but instead inherit the identity based versions
87     * from class <tt>Object</tt>, because element-based equality is not
88     * always well-defined for queues with the same elements but different
89     * ordering properties.
90     *
91     *
92   * <p>This interface is a member of the   * <p>This interface is a member of the
93   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/../guide/collections/index.html">
94   * Java Collections Framework</a>.   * Java Collections Framework</a>.
95   *   *
96   * @see Collection   * @see java.util.Collection
97   * @see LinkedList   * @see LinkedList
98   * @see PriorityQueue   * @see PriorityQueue
99   * @see java.util.concurrent.LinkedQueue   * @see java.util.concurrent.LinkedBlockingQueue
100   * @see java.util.concurrent.BlockingQueue   * @see java.util.concurrent.BlockingQueue
101   * @see java.util.concurrent.ArrayBlockingQueue   * @see java.util.concurrent.ArrayBlockingQueue
102   * @see java.util.concurrent.LinkedBlockingQueue   * @see java.util.concurrent.LinkedBlockingQueue
103   * @see java.util.concurrent.PriorityBlockingQueue   * @see java.util.concurrent.PriorityBlockingQueue
104   * @since 1.5   * @since 1.5
105   * @author Doug Lea   * @author Doug Lea
106     * @param <E> the type of elements held in this collection
107   */   */
108  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
109    
110      /**      /**
111       * Add the specified element to this queue, if possible.       * Inserts the specified element into this queue, if possible.  When
112         * using queues that may impose insertion restrictions (for
113         * example capacity bounds), method <tt>offer</tt> is generally
114         * preferable to method {@link Collection#add}, which can fail to
115         * insert an element only by throwing an exception.
116       *       *
117       * @param element the element to add.       * @param o the element to insert.
118       * @return true if it was possible to add the element to the queue.       * @return <tt>true</tt> if it was possible to add the element to
119         * this queue, else <tt>false</tt>
120       */       */
121      boolean offer(E element);      boolean offer(E o);
122    
123      /**      /**
124       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue, or <tt>null</tt>
125         * if this queue is empty.
126       *       *
127       * @return an element previously on the queue, or <tt>null</tt> if the       * @return the head of this queue, or <tt>null</tt> if this
128       *         queue is empty.       *         queue is empty.
129       */       */
130      E poll();      E poll();
131    
132      /**      /**
133       * Remove and return an element from the queue.  This method differs       * Retrieves and removes the head of this queue.  This method
134       * from the <tt>poll</tt> method in that it throws an exception if the       * differs from the <tt>poll</tt> method in that it throws an
135       * queue is empty.       * exception if this queue is empty.
136       *       *
137       * @return an element previously on the queue.       * @return the head of this queue.
138       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
139       */       */
140      E remove() throws NoSuchElementException;      E remove();
141    
142      /**      /**
143       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue,
144       * if the queue is empty.  This method returns the same object reference       * returning <tt>null</tt> if this queue is empty.
      * that would be returned by by the <tt>poll</tt> method.  The two methods  
      * differ in that this method does not remove the element from the queue.  
145       *       *
146       * @return an element on the queue, or <tt>null</tt> if the queue is empty.       * @return the head of this queue, or <tt>null</tt> if this queue
147         * is empty.
148       */       */
149      E peek();      E peek();
150    
151      /**      /**
152       * Return, but do not remove, an element from the queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
153       * differs from the <tt>peek</tt> method in that it throws an exception if       * differs from the <tt>peek</tt> method only in that it throws an
154       * the queue is empty.       * exception if this queue is empty.
155       *       *
156       * @return an element on the queue.       * @return the head of this queue.
157       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
158       */       */
159      E element() throws NoSuchElementException;      E element();
160  }  }

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