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revision 1.2, Sun May 18 18:10:02 2003 UTC revision 1.17, Fri Sep 12 15:38:26 2003 UTC
# Line 1  Line 1 
1    /*
2     * Written by Doug Lea with assistance from members of JCP JSR-166
3     * Expert Group and released to the public domain. Use, modify, and
4     * redistribute this code in any way without acknowledgement.
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.
13   *   *
14   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
15   * FIFO (first-in-first-out) manner.  Among the exceptions are priority   * FIFO (first-in-first-out) manner.  Among the exceptions are
16   * queues, which order elements according to a supplied comparators, or   * priority queues, which order elements according to a supplied
17   * the elements natural ordering.  Every Queue implementation must specify   * comparator, or the elements' natural ordering, and LIFO queues (or
18   * its ordering guarantees.   * stacks) which order the elements LIFO (last-in-first-out).
19   *   * Whatever the ordering used, the <em>head</em> of the queue is that
20   * <p>The {@link #offer(E)} method adds an element if possible, otherwise   * element which would be removed by a call to {@link #remove() } or
21   * returning <tt>false</tt>.  This differs from the {@link   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
22   * Collections#add(Object)} method, which throws an unchecked exception upon   * the <em> tail</em> of the queue. Other kinds of queues may use
23   * failure. It is designed for use in collections in which failure to   * different placement rules.  Every <tt>Queue</tt> implementation
24   * add is a normal, rather than exceptional occurrence, for example,   * must specify its ordering properties.
25   * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.   *
26   *   * <p>The {@link #offer offer} method inserts an element if possible,
27   * <p>The {@link #remove()} and {@link #poll()} methods remove and return an   * otherwise returning <tt>false</tt>.  This differs from the {@link
28   * element in accord with the implementation's ordering policy. For example,   * java.util.Collection#add Collection.add} method, which can fail to
29   * in FIFO queues, they remove and return the oldest element in the queue.   * add an element only by throwing an unchecked exception.  The
30   * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their   * <tt>offer</tt> method is designed for use when failure is a normal,
31   * behavior when the queue is empty: the <tt>remove()</tt> method throws an   * rather than exceptional occurrence, for example, in fixed-capacity
32   * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.   * (or &quot;bounded&quot;) queues.
33   *   *
34   * <p>The {@link #element()} and {@link #peek()} methods return but do   * <p>The {@link #remove()} and {@link #poll()} methods remove and
35   * not delete the element that would be obtained by a call to   * return the head of the queue.
36   * the <tt>remove</tt> and <tt>poll</tt> methods respectively.   * Exactly which element is removed from the queue is a
37     * function of the queue's ordering policy, which differs from
38     * implementation to implementation. The <tt>remove()</tt> and
39     * <tt>poll()</tt> methods differ only in their behavior when the
40     * queue is empty: the <tt>remove()</tt> method throws an exception,
41     * while the <tt>poll()</tt> method returns <tt>null</tt>.
42     *
43     * <p>The {@link #element()} and {@link #peek()} methods return, but do
44     * not remove, the head of the queue.
45   *   *
46   * <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
47   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
# Line 35  Line 49 
49   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
50   * extends this interface.   * extends this interface.
51   *   *
52   * <p><tt>Queue</tt> implementations generally do not allow insertion of   * <p><tt>Queue</tt> implementations generally do not allow insertion
53   * <tt>null</tt> elements.  Even in the few implementations that permit it,   * of <tt>null</tt> elements, although some implementations, such as
54   * it is a bad idea, as <tt>null</tt> is also used as a special return value   * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
55   * by the <tt>poll</tt> method to indicate that the queue contains no   * Even in the implementations that permit it, <tt>null</tt> should
56   * elements.   * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
57     * used as a special return value by the <tt>poll</tt> method to
58     * indicate that the queue contains no elements.
59     *
60     * <p><tt>Queue</tt> implementations generally do not define
61     * element-based versions of methods <tt>equals</tt> and
62     * <tt>hashCode</tt> but instead inherit the identity based versions
63     * from class <tt>Object</tt>, because element-based equality is not
64     * always well-defined for queues with the same elements but different
65     * ordering properties.
66     *
67   *   *
68   * <p>This interface is a member of the   * <p>This interface is a member of the
69   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/../guide/collections/index.html">
70   * Java Collections Framework</a>.   * Java Collections Framework</a>.
71   *   *
72   * @see Collection   * @see java.util.Collection
73   * @see LinkedList   * @see LinkedList
74   * @see PriorityQueue   * @see PriorityQueue
75   * @see LinkedQueue   * @see java.util.concurrent.LinkedBlockingQueue
76   * @see java.util.concurrent.BlockingQueue   * @see java.util.concurrent.BlockingQueue
77   * @see java.util.concurrent.ArrayBlockingQueue   * @see java.util.concurrent.ArrayBlockingQueue
78   * @see java.util.concurrent.LinkedBlockingQueue   * @see java.util.concurrent.LinkedBlockingQueue
79   * @see java.util.concurrent.PriorityBlockingQueue   * @see java.util.concurrent.PriorityBlockingQueue
80     * @since 1.5
81     * @author Doug Lea
82   */   */
83  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
84    
85      /**      /**
86       * Add the specified element to this queue, if possible.       * Inserts the specified element to this queue, if possible.  When
87       *       * using queues that may impose insertion restrictions (for
88       * @param element the element to add.       * example capacity bounds), method <tt>offer</tt> is generally
89       * @return true if it was possible to add the element to the queue.       * preferable to method {@link Collection#add}, which can fail to
90         * insert an element only by throwing an exception.
91         *
92         * @param o the element to insert.
93         * @return <tt>true</tt> if it was possible to add the element to
94         * this queue, else <tt>false</tt>
95       */       */
96      public boolean offer(E element);      boolean offer(E o);
97    
98      /**      /**
99       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue, if it is available.
      * Exactly which element is removed from the queue is a function  
      * of the queue's ordering policy, which differs from implementation  
      * to implementation.  Possible orderings include (but are not limited  
      * to) first-in-first-out (FIFO), element priority, and arbitrary.  
100       *       *
101       * @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
102       *         queue is empty.       *         queue is empty.
103       */       */
104      public E poll();      E poll();
105    
106      /**      /**
107       * Remove and return an element from the queue.  This method differs       * Retrieves and removes the head of this queue.
108       * from the <tt>poll</tt> method in that it throws an exception if the       * This method differs
109         * from the <tt>poll</tt> method in that it throws an exception if this
110       * queue is empty.       * queue is empty.
111       *       *
112       * @return an element previously on the queue.       * @return the head of this queue.
113       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
114       */       */
115      public E remove() throws NoSuchElementException;      E remove();
116    
117      /**      /**
118       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue.
119       * if the queue is empty.  This method returns the same object reference       * This method differs from the <tt>poll</tt>
120       * that would be returned by by the <tt>poll</tt> method.  The two methods       * method only in that this method does not remove the head element from
121       * differ in that this method does not remove the element from the queue.       * this queue.
122       *       *
123       * @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 is empty.
124       */       */
125      public E peek();      E peek();
126    
127      /**      /**
128       * Return, but do not remove, an element from the queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
129       * 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
130       * the queue is empty.       * exception if this queue is empty.
131       *       *
132       * @return an element on the queue.       * @return the head of this queue.
133       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
134       */       */
135      public E element() throws NoSuchElementException;      E element();
136  }  }

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