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revision 1.1, Wed May 14 21:30:45 2003 UTC revision 1.17, Fri Sep 12 15:38:26 2003 UTC
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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   * Queues are Collections supporting additional basic insertion,   * A collection designed for holding elements prior to processing.
11   * extraction, and inspection operations.   * Besides basic {@link java.util.Collection Collection} operations, queues provide
12   *   * additional insertion, extraction, and inspection operations.
13   * <p> Queues typically, but do not necessarily order elements in a   *
14   * FIFO (first-in-first-out) manner. Among the exceptions are priority   * <p>Queues typically, but do not necessarily, order elements in a
15   * queues, that order elements in accord with supplied   * FIFO (first-in-first-out) manner.  Among the exceptions are
16   * Comparators. Every Queue implementation must specify its ordering   * priority queues, which order elements according to a supplied
17   * guarantees,   * comparator, or the elements' natural ordering, and LIFO queues (or
18   *   * stacks) which order the elements LIFO (last-in-first-out).
19   * <p> The <tt>offer</tt> method adds an element if possible,   * Whatever the ordering used, the <em>head</em> of the queue is that
20   * otherwise returning <tt>false</tt>. This differs from the   * element which would be removed by a call to {@link #remove() } or
21   * Collections.add method, that throws an unchecked exception upon   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
22   * failure. It is designed for use in collections in which failure to   * the <em> tail</em> of the queue. Other kinds of queues may use
23   * add is a normal, rather than exceptional occurrence, for example,   * different placement rules.  Every <tt>Queue</tt> implementation
24   * in fixed-capacity queues.   * must specify its ordering properties.
25   *   *
26   * <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return   * <p>The {@link #offer offer} method inserts an element if possible,
27   * an element in accord with the implementation's ordering policies --   * otherwise returning <tt>false</tt>.  This differs from the {@link
28   * for example, in FIFO queues, it will return the oldest element.   * java.util.Collection#add Collection.add} method, which can fail to
29   * The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior   * add an element only by throwing an unchecked exception.  The
30   * when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while   * <tt>offer</tt> method is designed for use when failure is a normal,
31   * <tt>remove</tt> throws an exception. These are designed for usage   * rather than exceptional occurrence, for example, in fixed-capacity
32   * contexts in which emptiness is considered to be normal versus   * (or &quot;bounded&quot;) queues.
33   * exceptional.   *
34   *   * <p>The {@link #remove()} and {@link #poll()} methods remove and
35   * <p> The <tt>element</tt> and <tt>peek</tt> methods return but do   * return the head of the queue.
36   * not delete the element that would be obtained by a call to   * Exactly which element is removed from the queue is a
37   * <tt>remove</tt> and <tt>poll</tt> respectively.   * function of the queue's ordering policy, which differs from
38   *   * implementation to implementation. The <tt>remove()</tt> and
39   * <p> The Queue interface does not define blocking queue methods   * <tt>poll()</tt> methods differ only in their behavior when the
40   * (i.e., those that wait for elements to appear and/or for space to   * queue is empty: the <tt>remove()</tt> method throws an exception,
41   * be available) that are common in concurrent programming. These are   * while the <tt>poll()</tt> method returns <tt>null</tt>.
42   * defined in the extended java.util.concurrent.BlockingQueue   *
43   * interface.   * <p>The {@link #element()} and {@link #peek()} methods return, but do
44   *   * not remove, the head of the queue.
45   * <p> Queue implementations generally do not allow insertion of   *
46   * <tt>null</tt>. Even in those that allow it, it is a very bad idea   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
47   * to do so, since <tt>null</tt> is also used as a sentinel by   * methods</i>, which are common in concurrent programming.  These methods,
48   * <tt>poll</tt> to indicate that no elements exist.   * which wait for elements to appear or for space to become available, are
49   **/   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
50     * extends this interface.
51     *
52     * <p><tt>Queue</tt> implementations generally do not allow insertion
53     * of <tt>null</tt> elements, although some implementations, such as
54     * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
55     * Even in the implementations that permit it, <tt>null</tt> should
56     * 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
69     * <a href="{@docRoot}/../guide/collections/index.html">
70     * Java Collections Framework</a>.
71     *
72     * @see java.util.Collection
73     * @see LinkedList
74     * @see PriorityQueue
75     * @see java.util.concurrent.LinkedBlockingQueue
76     * @see java.util.concurrent.BlockingQueue
77     * @see java.util.concurrent.ArrayBlockingQueue
78     * @see java.util.concurrent.LinkedBlockingQueue
79     * @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 given object to this queue if possible.       * Inserts the specified element to this queue, if possible.  When
87       * @param x the object to add       * using queues that may impose insertion restrictions (for
88       * @return true if successful       * example capacity bounds), method <tt>offer</tt> is generally
89       **/       * preferable to method {@link Collection#add}, which can fail to
90      public boolean offer(E x);       * insert an element only by throwing an exception.
91         *
92      /**       * @param o the element to insert.
93       * Delete and return an object from the queue if one is available.       * @return <tt>true</tt> if it was possible to add the element to
94       * @return the object, or null if the queue is empty.       * this queue, else <tt>false</tt>
95       **/       */
96      public E poll();      boolean offer(E o);
97    
98      /**      /**
99       * Delete and return the element produced by poll, if the queue is       * Retrieves and removes the head of this queue, if it is available.
100       * not empty.       *
101       * @return an element       * @return the head of this queue, or <tt>null</tt> if this
102       * @throws NoSuchElementException if empty       *         queue is empty.
103       **/       */
104      public E remove() throws NoSuchElementException;      E poll();
105    
106      /**      /**
107       * Return but do not delete the element that will be returned by       * Retrieves and removes the head of this queue.
108       * the next call to poll.       * This method differs
109       * @return an element, or null if empty       * from the <tt>poll</tt> method in that it throws an exception if this
110       **/       * queue is empty.
111      public E peek();       *
112         * @return the head of this queue.
113      /**       * @throws NoSuchElementException if this queue is empty.
114       * Return but do not delete the element that will be returned by       */
115       * the next call to poll, if the queue is not empty.      E remove();
116       * @return an element  
117       * @throws NoSuchElementException if empty      /**
118       **/       * Retrieves, but does not remove, the head of this queue.
119      public E element() throws NoSuchElementException;       * This method differs from the <tt>poll</tt>
120         * method only in that this method does not remove the head element from
121         * this queue.
122         *
123         * @return the head of this queue, or <tt>null</tt> if this queue is empty.
124         */
125        E peek();
126    
127        /**
128         * Retrieves, but does not remove, the head of this queue.  This method
129         * differs from the <tt>peek</tt> method only in that it throws an
130         * exception if this queue is empty.
131         *
132         * @return the head of this queue.
133         * @throws NoSuchElementException if this queue is empty.
134         */
135        E element();
136  }  }

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