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revision 1.7, Tue Jun 24 14:34:30 2003 UTC revision 1.26, Mon May 2 04:19:58 2005 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,
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><table BORDER CELLPADDING=3 CELLSPACING=1>
22     *  <tr>
23     *    <td></td>
24     *    <td ALIGN=CENTER><em>Throws exception</em></td>
25     *    <td ALIGN=CENTER><em>Returns special value</em></td>
26     *  </tr>
27     *  <tr>
28     *    <td><b>Insert</b></td>
29     *    <td>{@link #add add(e)}</td>
30     *    <td>{@link #offer offer(e)}</td>
31     *  </tr>
32     *  <tr>
33     *    <td><b>Remove</b></td>
34     *    <td>{@link #remove remove()}</td>
35     *    <td>{@link #poll poll()}</td>
36     *  </tr>
37     *  <tr>
38     *    <td><b>Examine</b></td>
39     *    <td>{@link #element element()}</td>
40     *    <td>{@link #peek peek()}</td>
41     *  </tr>
42     * </table>
43     *
44   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
45   * FIFO (first-in-first-out) manner.  Among the exceptions are   * FIFO (first-in-first-out) manner.  Among the exceptions are
46   * priority queues, which order elements according to a supplied   * priority queues, which order elements according to a supplied
47   * comparator, or the elements' natural ordering.  Every Queue   * comparator, or the elements' natural ordering, and LIFO queues (or
48   * implementation must specify its ordering guarantees.   * stacks) which order the elements LIFO (last-in-first-out).
49   *   * Whatever the ordering used, the <em>head</em> of the queue is that
50   * <p>The {@link #offer(E)} method adds an element if possible, otherwise   * element which would be removed by a call to {@link #remove() } or
51   * returning <tt>false</tt>.  This differs from the {@link   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
52   * Collections#add(Object)} method, which throws an unchecked exception upon   * the <em> tail</em> of the queue. Other kinds of queues may use
53   * failure. It is designed for use in collections in which failure to   * different placement rules.  Every <tt>Queue</tt> implementation
54   * add is a normal, rather than exceptional occurrence, for example,   * must specify its ordering properties.
55   * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.   *
56     * <p>The {@link #offer offer} method inserts an element if possible,
57     * otherwise returning <tt>false</tt>.  This differs from the {@link
58     * java.util.Collection#add Collection.add} method, which can fail to
59     * add an element only by throwing an unchecked exception.  The
60     * <tt>offer</tt> method is designed for use when failure is a normal,
61     * rather than exceptional occurrence, for example, in fixed-capacity
62     * (or &quot;bounded&quot;) queues.
63   *   *
64   * <p>The {@link #remove()} and {@link #poll()} methods remove and   * <p>The {@link #remove()} and {@link #poll()} methods remove and
65   * return an element in accord with the implementation's ordering   * return the head of the queue.
66   * policy.  Exactly which element is removed from the queue is a   * Exactly which element is removed from the queue is a
67   * function of the queue's ordering policy, which differs from   * function of the queue's ordering policy, which differs from
68   * 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  
69   * <tt>poll()</tt> methods differ only in their behavior when the   * <tt>poll()</tt> methods differ only in their behavior when the
70   * queue is empty: the <tt>remove()</tt> method throws an exception,   * queue is empty: the <tt>remove()</tt> method throws an exception,
71   * while the <tt>poll()</tt> method returns <tt>null</tt>.   * while the <tt>poll()</tt> method returns <tt>null</tt>.
72   *   *
73   * <p>The {@link #element()} and {@link #peek()} methods return but do   * <p>The {@link #element()} and {@link #peek()} methods return, but do
74   * 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.  
75   *   *
76   * <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
77   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
# Line 54  Line 87 
87   * 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
88   * indicate that the queue contains no elements.   * indicate that the queue contains no elements.
89   *   *
90     * <p><tt>Queue</tt> implementations generally do not define
91     * element-based versions of methods <tt>equals</tt> and
92     * <tt>hashCode</tt> but instead inherit the identity based versions
93     * from class <tt>Object</tt>, because element-based equality is not
94     * always well-defined for queues with the same elements but different
95     * ordering properties.
96     *
97     *
98   * <p>This interface is a member of the   * <p>This interface is a member of the
99   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/../guide/collections/index.html">
100   * Java Collections Framework</a>.   * Java Collections Framework</a>.
101   *   *
102   * @see Collection   * @see java.util.Collection
103   * @see LinkedList   * @see LinkedList
104   * @see PriorityQueue   * @see PriorityQueue
105   * @see java.util.concurrent.LinkedQueue   * @see java.util.concurrent.LinkedBlockingQueue
106   * @see java.util.concurrent.BlockingQueue   * @see java.util.concurrent.BlockingQueue
107   * @see java.util.concurrent.ArrayBlockingQueue   * @see java.util.concurrent.ArrayBlockingQueue
108   * @see java.util.concurrent.LinkedBlockingQueue   * @see java.util.concurrent.LinkedBlockingQueue
109   * @see java.util.concurrent.PriorityBlockingQueue   * @see java.util.concurrent.PriorityBlockingQueue
110   * @since 1.5   * @since 1.5
111   * @author Doug Lea   * @author Doug Lea
112     * @param <E> the type of elements held in this collection
113   */   */
114  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
115    
116      /**      /**
117       * Add the specified element to this queue, if possible.       * Inserts the specified element into this queue, if possible.  When
118         * using queues that may impose insertion restrictions (for
119         * example capacity bounds), method <tt>offer</tt> is generally
120         * preferable to method {@link Collection#add}, which can fail to
121         * insert an element only by throwing an exception.
122       *       *
123       * @param element the element to add.       * @param o the element to insert.
124       * @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
125         * this queue, else <tt>false</tt>
126       */       */
127      boolean offer(E element);      boolean offer(E o);
128    
129      /**      /**
130       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue, or <tt>null</tt>
131         * if this queue is empty.
132       *       *
133       * @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
134       *         queue is empty.       *         queue is empty.
135       */       */
136      E poll();      E poll();
137    
138      /**      /**
139       * Remove and return an element from the queue.  This method differs       * Retrieves and removes the head of this queue.  This method
140       * 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
141       * queue is empty.       * exception if this queue is empty.
142       *       *
143       * @return an element previously on the queue.       * @return the head of this queue.
144       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
145       */       */
146      E remove() throws NoSuchElementException;      E remove();
147    
148      /**      /**
149       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue,
150       * 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.  
151       *       *
152       * @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
153         * is empty.
154       */       */
155      E peek();      E peek();
156    
157      /**      /**
158       * Return, but do not remove, an element from the queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
159       * 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
160       * the queue is empty.       * exception if this queue is empty.
161       *       *
162       * @return an element on the queue.       * @return the head of this queue.
163       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
164       */       */
165      E element() throws NoSuchElementException;      E element();
166  }  }

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