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revision 1.7, Tue Jun 24 14:34:30 2003 UTC revision 1.47, Sat May 6 06:55:50 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 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  0 *   * 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 style="text-align:center"><em>Throws exception</em></td>
25     *    <td style="text-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
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 {@code Queue} 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 {@code false}.  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     * {@code offer} 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 {@code remove()} and
69   * are not limited to) first-in-first-out (FIFO), last-in-first-out   * {@code poll()} methods differ only in their behavior when the
70   * (LIFO), element priority, and arbitrary.  The <tt>remove()</tt> and   * queue is empty: the {@code remove()} method throws an exception,
71   * <tt>poll()</tt> methods differ only in their behavior when the   * while the {@code poll()} method returns {@code null}.
  * queue is empty: the <tt>remove()</tt> method throws an exception,  
  * while the <tt>poll()</tt> method returns <tt>null</tt>.  
  *  
  * <p>The {@link #element()} and {@link #peek()} methods return but do  
  * not delete the element that would be obtained by a call to  
  * the <tt>remove</tt> and <tt>poll</tt> methods respectively.  
72   *   *
73   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue   * <p>The {@link #element()} and {@link #peek()} methods return, but do
74     * not remove, the head of the queue.
75     *
76     * <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>{@code Queue} implementations generally do not define
91     * element-based versions of methods {@code equals} and
92     * {@code hashCode} but instead inherit the identity based versions
93     * from class {@code Object}, because element-based equality is not
94     * always well-defined for queues with the same elements but different
95     * 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 Collection  
  * @see LinkedList  
  * @see PriorityQueue  
  * @see java.util.concurrent.LinkedQueue  
  * @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       * Add the specified element to this queue, if possible.       * Inserts the specified element into this queue if it is possible to do so
108       *       * immediately without violating capacity restrictions, returning
109       * @param element the element to add.       * {@code true} upon success and throwing an {@code IllegalStateException}
110       * @return true if it was possible to add the element to the queue.       * if no space is currently available.
111         *
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         * @return the head of this queue
150         * @throws NoSuchElementException if this queue is empty
151       */       */
152      boolean offer(E element);      E remove();
153    
154      /**      /**
155       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue,
156         * or returns {@code null} if this queue is empty.
157       *       *
158       * @return an element previously on the queue, or <tt>null</tt> if the       * @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       * Remove and return an element from the queue.  This method differs       * Retrieves, but does not remove, the head of this queue.  This method
164       * from the <tt>poll</tt> method in that it throws an exception if the       * differs from {@link #peek peek} only in that it throws an exception
165       * queue is empty.       * if this queue is empty.
166       *       *
167       * @return an element previously on the queue.       * @return the head of this queue
168       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty
169       */       */
170      E remove() throws NoSuchElementException;      E element();
171    
172      /**      /**
173       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue,
174       * if the queue is empty.  This method returns the same object reference       * or returns {@code null} 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.  
175       *       *
176       * @return an element on the queue, or <tt>null</tt> if the queue is empty.       * @return the head of this queue, or {@code null} if this queue is empty
177       */       */
178      E peek();      E peek();
   
     /**  
      * Return, but do not remove, an element from the queue.  This method  
      * differs from the <tt>peek</tt> method in that it throws an exception if  
      * the queue is empty.  
      *  
      * @return an element on the queue.  
      * @throws NoSuchElementException if the queue is empty.  
      */  
     E element() throws NoSuchElementException;  
179  }  }

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