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revision 1.7, Tue Jun 24 14:34:30 2003 UTC revision 1.44, Fri Jul 8 20:02:54 2016 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 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 {@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     * <table BORDER CELLPADDING=3 CELLSPACING=1>
22     * <caption>Summary of Queue methods</caption>
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 Queue#add add(e)}</td>
31     *    <td>{@link Queue#offer offer(e)}</td>
32     *  </tr>
33     *  <tr>
34     *    <td><b>Remove</b></td>
35     *    <td>{@link Queue#remove remove()}</td>
36     *    <td>{@link Queue#poll poll()}</td>
37     *  </tr>
38     *  <tr>
39     *    <td><b>Examine</b></td>
40     *    <td>{@link Queue#element element()}</td>
41     *    <td>{@link Queue#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 {@code Queue} 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 {@code false}.  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     * {@code offer} 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 {@code remove()} and
70   * are not limited to) first-in-first-out (FIFO), last-in-first-out   * {@code poll()} methods differ only in their behavior when the
71   * (LIFO), element priority, and arbitrary.  The <tt>remove()</tt> and   * queue is empty: the {@code remove()} method throws an exception,
72   * <tt>poll()</tt> methods differ only in their behavior when the   * while the {@code poll()} method returns {@code null}.
73   * queue is empty: the <tt>remove()</tt> method throws an exception,   *
74   * while the <tt>poll()</tt> method returns <tt>null</tt>.   * <p>The {@link #element()} and {@link #peek()} methods return, but do
75   *   * not remove, the head of the queue.
  * <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.  
76   *   *
77   * <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
78   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
79   * 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
80   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
81   * extends this interface.   * extends this interface.
82   *   *
83   * <p><tt>Queue</tt> implementations generally do not allow insertion   * <p>{@code Queue} implementations generally do not allow insertion
84   * of <tt>null</tt> elements, although some implementations, such as   * of {@code null} elements, although some implementations, such as
85   * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.   * {@link LinkedList}, do not prohibit insertion of {@code null}.
86   * Even in the implementations that permit it, <tt>null</tt> should   * Even in the implementations that permit it, {@code null} should
87   * 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
88   * 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
89   * indicate that the queue contains no elements.   * indicate that the queue contains no elements.
90   *   *
91     * <p>{@code Queue} implementations generally do not define
92     * element-based versions of methods {@code equals} and
93     * {@code hashCode} but instead inherit the identity based versions
94     * from class {@code Object}, because element-based equality is not
95     * always well-defined for queues with the same elements but different
96     * ordering properties.
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}/../technotes/guides/collections/index.html">
100   * Java Collections Framework</a>.   * Java Collections Framework</a>.
101   *   *
  * @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  
102   * @since 1.5   * @since 1.5
103   * @author Doug Lea   * @author Doug Lea
104     * @param <E> the type of elements held in this queue
105   */   */
106  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
107      /**      /**
108       * Add the specified element to this queue, if possible.       * Inserts the specified element into this queue if it is possible to do so
109       *       * immediately without violating capacity restrictions, returning
110       * @param element the element to add.       * {@code true} upon success and throwing an {@code IllegalStateException}
111       * @return true if it was possible to add the element to the queue.       * if no space is currently available.
112         *
113         * @param e the element to add
114         * @return {@code true} (as specified by {@link Collection#add})
115         * @throws IllegalStateException if the element cannot be added at this
116         *         time due to capacity restrictions
117         * @throws ClassCastException if the class of the specified element
118         *         prevents it from being added to this queue
119         * @throws NullPointerException if the specified element is null and
120         *         this queue does not permit null elements
121         * @throws IllegalArgumentException if some property of this element
122         *         prevents it from being added to this queue
123         */
124        boolean add(E e);
125    
126        /**
127         * Inserts the specified element into this queue if it is possible to do
128         * so immediately without violating capacity restrictions.
129         * When using a capacity-restricted queue, this method is generally
130         * preferable to {@link #add}, which can fail to insert an element only
131         * by throwing an exception.
132         *
133         * @param e the element to add
134         * @return {@code true} if the element was added to this queue, else
135         *         {@code false}
136         * @throws ClassCastException if the class of the specified element
137         *         prevents it from being added to this queue
138         * @throws NullPointerException if the specified element is null and
139         *         this queue does not permit null elements
140         * @throws IllegalArgumentException if some property of this element
141         *         prevents it from being added to this queue
142       */       */
143      boolean offer(E element);      boolean offer(E e);
144    
145      /**      /**
146       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue.  This method differs
147       *       * from {@link #poll poll} only in that it throws an exception if this
      * @return an element previously on the queue, or <tt>null</tt> if the  
148       *         queue is empty.       *         queue is empty.
149         *
150         * @return the head of this queue
151         * @throws NoSuchElementException if this queue is empty
152       */       */
153      E poll();      E remove();
154    
155      /**      /**
156       * Remove and return an element from the queue.  This method differs       * Retrieves and removes the head of this queue,
157       * from the <tt>poll</tt> method in that it throws an exception if the       * or returns {@code null} if this queue is empty.
      * queue is empty.  
158       *       *
159       * @return an element previously on the queue.       * @return the head of this queue, or {@code null} if this queue is empty
      * @throws NoSuchElementException if the queue is empty.  
160       */       */
161      E remove() throws NoSuchElementException;      E poll();
162    
163      /**      /**
164       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue.  This method
165       * if the queue is empty.  This method returns the same object reference       * differs from {@link #peek peek} only in that it throws an exception
166       * that would be returned by by the <tt>poll</tt> method.  The two methods       * if this queue is empty.
      * differ in that this method does not remove the element from the queue.  
167       *       *
168       * @return an element on the queue, or <tt>null</tt> if the queue is empty.       * @return the head of this queue
169         * @throws NoSuchElementException if this queue is empty
170       */       */
171      E peek();      E element();
172    
173      /**      /**
174       * Return, but do not remove, an element from the queue.  This method       * Retrieves, but does not remove, the head of this queue,
175       * differs from the <tt>peek</tt> method in that it throws an exception if       * or returns {@code null} if this queue is empty.
      * the queue is empty.  
176       *       *
177       * @return an element on the queue.       * @return the head of this queue, or {@code null} if this queue is empty
      * @throws NoSuchElementException if the queue is empty.  
178       */       */
179      E element() throws NoSuchElementException;      E peek();
180  }  }

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