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revision 1.7, Tue Jun 24 14:34:30 2003 UTC revision 1.28, Mon May 2 17:34:02 2005 UTC
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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,
12   * additional insertion, extraction, and inspection operations.   * queues provide additional insertion, extraction, and inspection
13  0 *   * 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
47   * priority queues, which order elements according to a supplied   * priority queues, which order elements according to a supplied
48   * comparator, or the elements' natural ordering.  Every Queue   * comparator, or the elements' natural ordering, and LIFO queues (or
49   * implementation must specify its ordering guarantees.   * stacks) which order the elements LIFO (last-in-first-out).
50   *   * Whatever the ordering used, the <em>head</em> of the queue is that
51   * <p>The {@link #offer(E)} method adds an element if possible, otherwise   * element which would be removed by a call to {@link #remove() } or
52   * returning <tt>false</tt>.  This differs from the {@link   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
53   * Collections#add(Object)} method, which throws an unchecked exception upon   * the <em> tail</em> of the queue. Other kinds of queues may use
54   * failure. It is designed for use in collections in which failure to   * different placement rules.  Every <tt>Queue</tt> implementation
55   * add is a normal, rather than exceptional occurrence, for example,   * must specify its ordering properties.
56   * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.   *
57     * <p>The {@link #offer offer} method inserts an element if possible,
58     * otherwise returning <tt>false</tt>.  This differs from the {@link
59     * java.util.Collection#add Collection.add} method, which can fail to
60     * add an element only by throwing an unchecked exception.  The
61     * <tt>offer</tt> method is designed for use when failure is a normal,
62     * rather than exceptional occurrence, for example, in fixed-capacity
63     * (or &quot;bounded&quot;) queues.
64   *   *
65   * <p>The {@link #remove()} and {@link #poll()} methods remove and   * <p>The {@link #remove()} and {@link #poll()} methods remove and
66   * return an element in accord with the implementation's ordering   * return the head of the queue.
67   * policy.  Exactly which element is removed from the queue is a   * Exactly which element is removed from the queue is a
68   * function of the queue's ordering policy, which differs from   * function of the queue's ordering policy, which differs from
69   * 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  
70   * <tt>poll()</tt> methods differ only in their behavior when the   * <tt>poll()</tt> methods differ only in their behavior when the
71   * queue is empty: the <tt>remove()</tt> method throws an exception,   * queue is empty: the <tt>remove()</tt> method throws an exception,
72   * while the <tt>poll()</tt> method returns <tt>null</tt>.   * while the <tt>poll()</tt> method returns <tt>null</tt>.
73   *   *
74   * <p>The {@link #element()} and {@link #peek()} methods return but do   * <p>The {@link #element()} and {@link #peek()} methods return, but do
75   * 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.  
76   *   *
77   * <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
78   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
# Line 54  Line 88 
88   * 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
89   * indicate that the queue contains no elements.   * indicate that the queue contains no elements.
90   *   *
91     * <p><tt>Queue</tt> implementations generally do not define
92     * element-based versions of methods <tt>equals</tt> and
93     * <tt>hashCode</tt> but instead inherit the identity based versions
94     * from class <tt>Object</tt>, because element-based equality is not
95     * always well-defined for queues with the same elements but different
96     * ordering properties.
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}/../guide/collections/index.html">
101   * Java Collections Framework</a>.   * Java Collections Framework</a>.
102   *   *
103   * @see Collection   * @see java.util.Collection
104   * @see LinkedList   * @see LinkedList
105   * @see PriorityQueue   * @see PriorityQueue
106   * @see java.util.concurrent.LinkedQueue   * @see java.util.concurrent.LinkedBlockingQueue
107   * @see java.util.concurrent.BlockingQueue   * @see java.util.concurrent.BlockingQueue
108   * @see java.util.concurrent.ArrayBlockingQueue   * @see java.util.concurrent.ArrayBlockingQueue
109   * @see java.util.concurrent.LinkedBlockingQueue   * @see java.util.concurrent.LinkedBlockingQueue
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       * Add the specified element to this queue, if possible.       * Inserts the specified element into this queue, if possible.  When
119         * using queues that may impose insertion restrictions (for
120         * example capacity bounds), method <tt>offer</tt> is generally
121         * preferable to method {@link Collection#add}, which can fail to
122         * insert an element only by throwing an exception.
123       *       *
124       * @param element the element to add.       * @param e the element to insert.
125       * @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
126         * this queue, else <tt>false</tt>
127       */       */
128      boolean offer(E element);      boolean offer(E e);
129    
130      /**      /**
131       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue, or <tt>null</tt>
132         * if this queue is empty.
133       *       *
134       * @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
135       *         queue is empty.       *         queue is empty.
136       */       */
137      E poll();      E poll();
138    
139      /**      /**
140       * Remove and return an element from the queue.  This method differs       * Retrieves and removes the head of this queue.  This method
141       * from the <tt>poll</tt> method in that it throws an exception if the       * differs from the {@link #poll} method only in that it throws an
142       * queue is empty.       * exception if this queue is empty.
143       *       *
144       * @return an element previously on the queue.       * @return the head of this queue.
145       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
146       */       */
147      E remove() throws NoSuchElementException;      E remove();
148    
149      /**      /**
150       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue,
151       * 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.  
152       *       *
153       * @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
154         * is empty.
155       */       */
156      E peek();      E peek();
157    
158      /**      /**
159       * Return, but do not remove, an element from the queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
160       * differs from the <tt>peek</tt> method in that it throws an exception if       * differs from the {@link #peek} method only in that it throws an
161       * the queue is empty.       * exception if this queue is empty.
162       *       *
163       * @return an element on the queue.       * @return the head of this queue.
164       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
165       */       */
166      E element() throws NoSuchElementException;      E element();
167  }  }

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