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revision 1.17, Fri Sep 12 15:38:26 2003 UTC revision 1.40, Sat Jul 20 18:00:11 2013 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/publicdomain/zero/1.0/
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 java.util.Collection 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     * 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 {@code null} or {@code false}, depending on the
16     * operation).  The latter form of the insert operation is designed
17     * specifically for use with capacity-restricted {@code Queue}
18     * implementations; in most implementations, insert operations cannot
19     * fail.
20     *
21     * <p>
22     * <table BORDER CELLPADDING=3 CELLSPACING=1>
23     * <caption>Summary of Queue methods</caption>
24     *  <tr>
25     *    <td></td>
26     *    <td ALIGN=CENTER><em>Throws exception</em></td>
27     *    <td ALIGN=CENTER><em>Returns special value</em></td>
28     *  </tr>
29     *  <tr>
30     *    <td><b>Insert</b></td>
31     *    <td>{@link Queue#add add(e)}</td>
32     *    <td>{@link Queue#offer offer(e)}</td>
33     *  </tr>
34     *  <tr>
35     *    <td><b>Remove</b></td>
36     *    <td>{@link Queue#remove remove()}</td>
37     *    <td>{@link Queue#poll poll()}</td>
38     *  </tr>
39     *  <tr>
40     *    <td><b>Examine</b></td>
41     *    <td>{@link Queue#element element()}</td>
42     *    <td>{@link Queue#peek peek()}</td>
43     *  </tr>
44     * </table>
45   *   *
46   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
47   * FIFO (first-in-first-out) manner.  Among the exceptions are   * FIFO (first-in-first-out) manner.  Among the exceptions are
# Line 20  Line 52 
52   * element which would be removed by a call to {@link #remove() } or   * element which would be removed by a call to {@link #remove() } or
53   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
54   * the <em> tail</em> of the queue. Other kinds of queues may use   * the <em> tail</em> of the queue. Other kinds of queues may use
55   * different placement rules.  Every <tt>Queue</tt> implementation   * different placement rules.  Every {@code Queue} implementation
56   * must specify its ordering properties.   * must specify its ordering properties.
57   *   *
58   * <p>The {@link #offer offer} method inserts an element if possible,   * <p>The {@link #offer offer} method inserts an element if possible,
59   * otherwise returning <tt>false</tt>.  This differs from the {@link   * otherwise returning {@code false}.  This differs from the {@link
60   * java.util.Collection#add Collection.add} method, which can fail to   * java.util.Collection#add Collection.add} method, which can fail to
61   * add an element only by throwing an unchecked exception.  The   * add an element only by throwing an unchecked exception.  The
62   * <tt>offer</tt> method is designed for use when failure is a normal,   * {@code offer} method is designed for use when failure is a normal,
63   * rather than exceptional occurrence, for example, in fixed-capacity   * rather than exceptional occurrence, for example, in fixed-capacity
64   * (or &quot;bounded&quot;) queues.   * (or &quot;bounded&quot;) queues.
65   *   *
# Line 35  Line 67 
67   * return the head of the queue.   * return the head of the queue.
68   * Exactly which element is removed from the queue is a   * Exactly which element is removed from the queue is a
69   * function of the queue's ordering policy, which differs from   * function of the queue's ordering policy, which differs from
70   * implementation to implementation. The <tt>remove()</tt> and   * implementation to implementation. The {@code remove()} and
71   * <tt>poll()</tt> methods differ only in their behavior when the   * {@code poll()} methods differ only in their behavior when the
72   * queue is empty: the <tt>remove()</tt> method throws an exception,   * queue is empty: the {@code remove()} method throws an exception,
73   * while the <tt>poll()</tt> method returns <tt>null</tt>.   * while the {@code poll()} method returns {@code null}.
74   *   *
75   * <p>The {@link #element()} and {@link #peek()} methods return, but do   * <p>The {@link #element()} and {@link #peek()} methods return, but do
76   * not remove, the head of the queue.   * not remove, the head of the queue.
77   *   *
78   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue   * <p>The {@code Queue} interface does not define the <i>blocking queue
79   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
80   * which wait for elements to appear or for space to become available, are   * which wait for elements to appear or for space to become available, are
81   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
82   * extends this interface.   * extends this interface.
83   *   *
84   * <p><tt>Queue</tt> implementations generally do not allow insertion   * <p>{@code Queue} implementations generally do not allow insertion
85   * of <tt>null</tt> elements, although some implementations, such as   * of {@code null} elements, although some implementations, such as
86   * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.   * {@link LinkedList}, do not prohibit insertion of {@code null}.
87   * Even in the implementations that permit it, <tt>null</tt> should   * Even in the implementations that permit it, {@code null} should
88   * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also   * not be inserted into a {@code Queue}, as {@code null} is also
89   * used as a special return value by the <tt>poll</tt> method to   * used as a special return value by the {@code poll} method to
90   * indicate that the queue contains no elements.   * indicate that the queue contains no elements.
91   *   *
92   * <p><tt>Queue</tt> implementations generally do not define   * <p>{@code Queue} implementations generally do not define
93   * element-based versions of methods <tt>equals</tt> and   * element-based versions of methods {@code equals} and
94   * <tt>hashCode</tt> but instead inherit the identity based versions   * {@code hashCode} but instead inherit the identity based versions
95   * from class <tt>Object</tt>, because element-based equality is not   * from class {@code Object}, because element-based equality is not
96   * always well-defined for queues with the same elements but different   * always well-defined for queues with the same elements but different
97   * ordering properties.   * ordering properties.
98   *   *
99   *   *
100   * <p>This interface is a member of the   * <p>This interface is a member of the
101   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
102   * Java Collections Framework</a>.   * Java Collections Framework</a>.
103   *   *
104   * @see java.util.Collection   * @see java.util.Collection
# Line 79  Line 111 
111   * @see java.util.concurrent.PriorityBlockingQueue   * @see java.util.concurrent.PriorityBlockingQueue
112   * @since 1.5   * @since 1.5
113   * @author Doug Lea   * @author Doug Lea
114     * @param <E> the type of elements held in this collection
115   */   */
116  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
   
     /**  
      * Inserts the specified element to this queue, if possible.  When  
      * using queues that may impose insertion restrictions (for  
      * example capacity bounds), method <tt>offer</tt> is generally  
      * preferable to method {@link Collection#add}, which can fail to  
      * insert an element only by throwing an exception.  
      *  
      * @param o the element to insert.  
      * @return <tt>true</tt> if it was possible to add the element to  
      * this queue, else <tt>false</tt>  
      */  
     boolean offer(E o);  
   
117      /**      /**
118       * Retrieves and removes the head of this queue, if it is available.       * Inserts the specified element into this queue if it is possible to do so
119       *       * immediately without violating capacity restrictions, returning
120       * @return the head of this queue, or <tt>null</tt> if this       * {@code true} upon success and throwing an {@code IllegalStateException}
121       *         queue is empty.       * if no space is currently available.
122         *
123         * @param e the element to add
124         * @return {@code true} (as specified by {@link Collection#add})
125         * @throws IllegalStateException if the element cannot be added at this
126         *         time due to capacity restrictions
127         * @throws ClassCastException if the class of the specified element
128         *         prevents it from being added to this queue
129         * @throws NullPointerException if the specified element is null and
130         *         this queue does not permit null elements
131         * @throws IllegalArgumentException if some property of this element
132         *         prevents it from being added to this queue
133         */
134        boolean add(E e);
135    
136        /**
137         * Inserts the specified element into this queue if it is possible to do
138         * so immediately without violating capacity restrictions.
139         * When using a capacity-restricted queue, this method is generally
140         * preferable to {@link #add}, which can fail to insert an element only
141         * by throwing an exception.
142         *
143         * @param e the element to add
144         * @return {@code true} if the element was added to this queue, else
145         *         {@code false}
146         * @throws ClassCastException if the class of the specified element
147         *         prevents it from being added to this queue
148         * @throws NullPointerException if the specified element is null and
149         *         this queue does not permit null elements
150         * @throws IllegalArgumentException if some property of this element
151         *         prevents it from being added to this queue
152       */       */
153      E poll();      boolean offer(E e);
154    
155      /**      /**
156       * Retrieves and removes the head of this queue.       * Retrieves and removes the head of this queue.  This method differs
157       * This method differs       * from {@link #poll poll} only in that it throws an exception if this
      * from the <tt>poll</tt> method in that it throws an exception if this  
158       * queue is empty.       * queue is empty.
159       *       *
160       * @return the head of this queue.       * @return the head of this queue
161       * @throws NoSuchElementException if this queue is empty.       * @throws NoSuchElementException if this queue is empty
162       */       */
163      E remove();      E remove();
164    
165      /**      /**
166       * Retrieves, but does not remove, the head of this queue.       * Retrieves and removes the head of this queue,
167       * This method differs from the <tt>poll</tt>       * or returns {@code null} if this queue is empty.
      * method only in that this method does not remove the head element from  
      * this queue.  
168       *       *
169       * @return the head of this queue, or <tt>null</tt> if this queue is empty.       * @return the head of this queue, or {@code null} if this queue is empty
170       */       */
171      E peek();      E poll();
172    
173      /**      /**
174       * Retrieves, but does not remove, the head of this queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
175       * differs from the <tt>peek</tt> method only in that it throws an       * differs from {@link #peek peek} only in that it throws an exception
176       * exception if this queue is empty.       * if this queue is empty.
177       *       *
178       * @return the head of this queue.       * @return the head of this queue
179       * @throws NoSuchElementException if this queue is empty.       * @throws NoSuchElementException if this queue is empty
180       */       */
181      E element();      E element();
182    
183        /**
184         * Retrieves, but does not remove, the head of this queue,
185         * or returns {@code null} if this queue is empty.
186         *
187         * @return the head of this queue, or {@code null} if this queue is empty
188         */
189        E peek();
190  }  }

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