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revision 1.19, Mon Sep 15 12:02:23 2003 UTC revision 1.46, Sat May 6 06:49:46 2017 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 Collection} operations, queues provide
12   * additional insertion, extraction, and inspection operations.   * additional insertion, extraction, and inspection operations.
13     * Each of these methods exists in two forms: one throws an exception
14     * if the operation fails, the other returns a special value (either
15     * {@code null} or {@code false}, depending on the operation).  The
16     * latter form of the insert operation is designed specifically for
17     * use with capacity-restricted {@code Queue} implementations; in most
18     * implementations, insert operations cannot fail.
19     *
20     * <table BORDER CELLPADDING=3 CELLSPACING=1>
21     * <caption>Summary of Queue methods</caption>
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(Object) add(e)}</td>
30     *    <td>{@link #offer(Object) 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
# Line 20  Line 50 
50   * element which would be removed by a call to {@link #remove() } or   * element which would be removed by a call to {@link #remove() } or
51   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
52   * 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
53   * different placement rules.  Every <tt>Queue</tt> implementation   * different placement rules.  Every {@code Queue} implementation
54   * must specify its ordering properties.   * must specify its ordering properties.
55   *   *
56   * <p>The {@link #offer offer} method inserts an element if possible,   * <p>The {@link #offer offer} method inserts an element if possible,
57   * otherwise returning <tt>false</tt>.  This differs from the {@link   * otherwise returning {@code false}.  This differs from the {@link
58   * java.util.Collection#add Collection.add} method, which can fail to   * java.util.Collection#add Collection.add} method, which can fail to
59   * add an element only by throwing an unchecked exception.  The   * add an element only by throwing an unchecked exception.  The
60   * <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,
61   * rather than exceptional occurrence, for example, in fixed-capacity   * rather than exceptional occurrence, for example, in fixed-capacity
62   * (or &quot;bounded&quot;) queues.   * (or &quot;bounded&quot;) queues.
63   *   *
# Line 35  Line 65 
65   * return the head of the queue.   * return the head of the queue.
66   * 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. The <tt>remove()</tt> and   * implementation to implementation. The {@code remove()} and
69   * <tt>poll()</tt> methods differ only in their behavior when the   * {@code poll()} methods differ only in their behavior when the
70   * queue is empty: the <tt>remove()</tt> method throws an exception,   * queue is empty: the {@code remove()} method throws an exception,
71   * while the <tt>poll()</tt> method returns <tt>null</tt>.   * while the {@code poll()} method returns {@code null}.
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 remove, the head of the queue.   * not remove, the head of the queue.
75   *   *
76   * <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
77   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
78   * 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
79   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
80   * extends this interface.   * extends this interface.
81   *   *
82   * <p><tt>Queue</tt> implementations generally do not allow insertion   * <p>{@code Queue} implementations generally do not allow insertion
83   * of <tt>null</tt> elements, although some implementations, such as   * of {@code null} elements, although some implementations, such as
84   * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.   * {@link LinkedList}, do not prohibit insertion of {@code null}.
85   * Even in the implementations that permit it, <tt>null</tt> should   * Even in the implementations that permit it, {@code null} should
86   * 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
87   * 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
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   * <p>{@code Queue} implementations generally do not define
91   * element-based versions of methods <tt>equals</tt> and   * element-based versions of methods {@code equals} and
92   * <tt>hashCode</tt> but instead inherit the identity based versions   * {@code hashCode} but instead inherit the identity based versions
93   * from class <tt>Object</tt>, because element-based equality is not   * from class {@code Object}, because element-based equality is not
94   * always well-defined for queues with the same elements but different   * always well-defined for queues with the same elements but different
95   * ordering properties.   * ordering properties.
96   *   *
  *  
97   * <p>This interface is a member of the   * <p>This interface is a member of the
98   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
99   * Java Collections Framework</a>.   * Java Collections Framework</a>.
100   *   *
  * @see java.util.Collection  
  * @see LinkedList  
  * @see PriorityQueue  
  * @see java.util.concurrent.LinkedBlockingQueue  
  * @see java.util.concurrent.BlockingQueue  
  * @see java.util.concurrent.ArrayBlockingQueue  
  * @see java.util.concurrent.LinkedBlockingQueue  
  * @see java.util.concurrent.PriorityBlockingQueue  
101   * @since 1.5   * @since 1.5
102   * @author Doug Lea   * @author Doug Lea
103     * @param <E> the type of elements held in this queue
104   */   */
105  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
   
106      /**      /**
107       * Inserts the specified element into this queue, if possible.  When       * Inserts the specified element into this queue if it is possible to do so
108       * using queues that may impose insertion restrictions (for       * immediately without violating capacity restrictions, returning
109       * example capacity bounds), method <tt>offer</tt> is generally       * {@code true} upon success and throwing an {@code IllegalStateException}
110       * preferable to method {@link Collection#add}, which can fail to       * if no space is currently available.
111       * insert an element only by throwing an exception.       *
112         * @param e the element to add
113         * @return {@code true} (as specified by {@link Collection#add})
114         * @throws IllegalStateException if the element cannot be added at this
115         *         time due to capacity restrictions
116         * @throws ClassCastException if the class of the specified element
117         *         prevents it from being added to this queue
118         * @throws NullPointerException if the specified element is null and
119         *         this queue does not permit null elements
120         * @throws IllegalArgumentException if some property of this element
121         *         prevents it from being added to this queue
122         */
123        boolean add(E e);
124    
125        /**
126         * Inserts the specified element into this queue if it is possible to do
127         * so immediately without violating capacity restrictions.
128         * When using a capacity-restricted queue, this method is generally
129         * preferable to {@link #add}, which can fail to insert an element only
130         * by throwing an exception.
131         *
132         * @param e the element to add
133         * @return {@code true} if the element was added to this queue, else
134         *         {@code false}
135         * @throws ClassCastException if the class of the specified element
136         *         prevents it from being added to this queue
137         * @throws NullPointerException if the specified element is null and
138         *         this queue does not permit null elements
139         * @throws IllegalArgumentException if some property of this element
140         *         prevents it from being added to this queue
141         */
142        boolean offer(E e);
143    
144        /**
145         * Retrieves and removes the head of this queue.  This method differs
146         * from {@link #poll() poll()} only in that it throws an exception if
147         * this queue is empty.
148       *       *
149       * @param o the element to insert.       * @return the head of this queue
150       * @return <tt>true</tt> if it was possible to add the element to       * @throws NoSuchElementException if this queue is empty
      * this queue, else <tt>false</tt>  
151       */       */
152      boolean offer(E o);      E remove();
153    
154      /**      /**
155       * Retrieves and removes the head of this queue, or <tt>null</tt>       * Retrieves and removes the head of this queue,
156       * if this queue is empty.       * or returns {@code null} if this queue is empty.
157       *       *
158       * @return the head of this queue, or <tt>null</tt> if this       * @return the head of this queue, or {@code null} if this queue is empty
      *         queue is empty.  
159       */       */
160      E poll();      E poll();
161    
162      /**      /**
163       * Retrieves and removes the head of this queue.  This method       * Retrieves, but does not remove, the head of this queue.  This method
164       * differs from the <tt>poll</tt> method in that it throws an       * differs from {@link #peek peek} only in that it throws an exception
165       * exception if this queue is empty.       * if this queue is empty.
166       *       *
167       * @return the head of this queue.       * @return the head of this queue
168       * @throws NoSuchElementException if this queue is empty.       * @throws NoSuchElementException if this queue is empty
169       */       */
170      E remove();      E element();
171    
172      /**      /**
173       * Retrieves, but does not remove, the head of this queue,       * Retrieves, but does not remove, the head of this queue,
174       * returning <tt>null</tt> if this queue is empty.       * or returns {@code null} if this queue is empty.
175       *       *
176       * @return the head of this queue, or <tt>null</tt> if this queue       * @return the head of this queue, or {@code null} if this queue is empty
      * is empty.  
177       */       */
178      E peek();      E peek();
   
     /**  
      * Retrieves, but does not remove, the head of this queue.  This method  
      * differs from the <tt>peek</tt> method only in that it throws an  
      * exception if this queue is empty.  
      *  
      * @return the head of this queue.  
      * @throws NoSuchElementException if this queue is empty.  
      */  
     E element();  
179  }  }
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