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revision 1.7, Tue Jun 24 14:34:30 2003 UTC revision 1.49, Wed Aug 23 20:17:53 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 class="striped">
21     * <caption>Summary of Queue methods</caption>
22     *  <thead>
23     *  <tr>
24     *    <td></td>
25     *    <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
26     *    <th scope="col" style="font-weight:normal; font-style:italic">Returns special value</th>
27     *  </tr>
28     *  </thead>
29     *  <tbody>
30     *  <tr>
31     *    <th scope="row">Insert</th>
32     *    <td>{@link #add(Object) add(e)}</td>
33     *    <td>{@link #offer(Object) offer(e)}</td>
34     *  </tr>
35     *  <tr>
36     *    <th scope="row">Remove</th>
37     *    <td>{@link #remove() remove()}</td>
38     *    <td>{@link #poll() poll()}</td>
39     *  </tr>
40     *  <tr>
41     *    <th scope="row">Examine</th>
42     *    <td>{@link #element() element()}</td>
43     *    <td>{@link #peek() peek()}</td>
44     *  </tr>
45     *  </tbody>
46     * </table>
47     *
48   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
49   * FIFO (first-in-first-out) manner.  Among the exceptions are   * FIFO (first-in-first-out) manner.  Among the exceptions are
50   * priority queues, which order elements according to a supplied   * priority queues, which order elements according to a supplied
51   * comparator, or the elements' natural ordering.  Every Queue   * comparator, or the elements' natural ordering, and LIFO queues (or
52   * implementation must specify its ordering guarantees.   * stacks) which order the elements LIFO (last-in-first-out).
53   *   * Whatever the ordering used, the <em>head</em> of the queue is that
54   * <p>The {@link #offer(E)} method adds an element if possible, otherwise   * element which would be removed by a call to {@link #remove()} or
55   * returning <tt>false</tt>.  This differs from the {@link   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
56   * Collections#add(Object)} method, which throws an unchecked exception upon   * the <em>tail</em> of the queue. Other kinds of queues may use
57   * failure. It is designed for use in collections in which failure to   * different placement rules.  Every {@code Queue} implementation
58   * add is a normal, rather than exceptional occurrence, for example,   * must specify its ordering properties.
59   * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.   *
60     * <p>The {@link #offer offer} method inserts an element if possible,
61     * otherwise returning {@code false}.  This differs from the {@link
62     * java.util.Collection#add Collection.add} method, which can fail to
63     * add an element only by throwing an unchecked exception.  The
64     * {@code offer} method is designed for use when failure is a normal,
65     * rather than exceptional occurrence, for example, in fixed-capacity
66     * (or &quot;bounded&quot;) queues.
67   *   *
68   * <p>The {@link #remove()} and {@link #poll()} methods remove and   * <p>The {@link #remove()} and {@link #poll()} methods remove and
69   * return an element in accord with the implementation's ordering   * return the head of the queue.
70   * policy.  Exactly which element is removed from the queue is a   * Exactly which element is removed from the queue is a
71   * function of the queue's ordering policy, which differs from   * function of the queue's ordering policy, which differs from
72   * implementation to implementation.  Possible orderings include (but   * implementation to implementation. The {@code remove()} and
73   * are not limited to) first-in-first-out (FIFO), last-in-first-out   * {@code poll()} methods differ only in their behavior when the
74   * (LIFO), element priority, and arbitrary.  The <tt>remove()</tt> and   * queue is empty: the {@code remove()} method throws an exception,
75   * <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.  
76   *   *
77   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue   * <p>The {@link #element()} and {@link #peek()} methods return, but do
78     * not remove, the head of the queue.
79     *
80     * <p>The {@code Queue} interface does not define the <i>blocking queue
81   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
82   * 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
83   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
84   * extends this interface.   * extends this interface.
85   *   *
86   * <p><tt>Queue</tt> implementations generally do not allow insertion   * <p>{@code Queue} implementations generally do not allow insertion
87   * of <tt>null</tt> elements, although some implementations, such as   * of {@code null} elements, although some implementations, such as
88   * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.   * {@link LinkedList}, do not prohibit insertion of {@code null}.
89   * Even in the implementations that permit it, <tt>null</tt> should   * Even in the implementations that permit it, {@code null} should
90   * 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
91   * 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
92   * indicate that the queue contains no elements.   * indicate that the queue contains no elements.
93   *   *
94     * <p>{@code Queue} implementations generally do not define
95     * element-based versions of methods {@code equals} and
96     * {@code hashCode} but instead inherit the identity based versions
97     * from class {@code Object}, because element-based equality is not
98     * always well-defined for queues with the same elements but different
99     * ordering properties.
100     *
101   * <p>This interface is a member of the   * <p>This interface is a member of the
102   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
103   * Java Collections Framework</a>.   * Java Collections Framework</a>.
104   *   *
  * @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  
105   * @since 1.5   * @since 1.5
106   * @author Doug Lea   * @author Doug Lea
107     * @param <E> the type of elements held in this queue
108   */   */
109  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
110      /**      /**
111       * Add the specified element to this queue, if possible.       * Inserts the specified element into this queue if it is possible to do so
112       *       * immediately without violating capacity restrictions, returning
113       * @param element the element to add.       * {@code true} upon success and throwing an {@code IllegalStateException}
114       * @return true if it was possible to add the element to the queue.       * if no space is currently available.
115         *
116         * @param e the element to add
117         * @return {@code true} (as specified by {@link Collection#add})
118         * @throws IllegalStateException if the element cannot be added at this
119         *         time due to capacity restrictions
120         * @throws ClassCastException if the class of the specified element
121         *         prevents it from being added to this queue
122         * @throws NullPointerException if the specified element is null and
123         *         this queue does not permit null elements
124         * @throws IllegalArgumentException if some property of this element
125         *         prevents it from being added to this queue
126         */
127        boolean add(E e);
128    
129        /**
130         * Inserts the specified element into this queue if it is possible to do
131         * so immediately without violating capacity restrictions.
132         * When using a capacity-restricted queue, this method is generally
133         * preferable to {@link #add}, which can fail to insert an element only
134         * by throwing an exception.
135         *
136         * @param e the element to add
137         * @return {@code true} if the element was added to this queue, else
138         *         {@code false}
139         * @throws ClassCastException if the class of the specified element
140         *         prevents it from being added to this queue
141         * @throws NullPointerException if the specified element is null and
142         *         this queue does not permit null elements
143         * @throws IllegalArgumentException if some property of this element
144         *         prevents it from being added to this queue
145         */
146        boolean offer(E e);
147    
148        /**
149         * Retrieves and removes the head of this queue.  This method differs
150         * from {@link #poll() poll()} only in that it throws an exception if
151         * this queue is empty.
152         *
153         * @return the head of this queue
154         * @throws NoSuchElementException if this queue is empty
155       */       */
156      boolean offer(E element);      E remove();
157    
158      /**      /**
159       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue,
160         * or returns {@code null} if this queue is empty.
161       *       *
162       * @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.  
163       */       */
164      E poll();      E poll();
165    
166      /**      /**
167       * Remove and return an element from the queue.  This method differs       * Retrieves, but does not remove, the head of this queue.  This method
168       * 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
169       * queue is empty.       * if this queue is empty.
170       *       *
171       * @return an element previously on the queue.       * @return the head of this queue
172       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty
173       */       */
174      E remove() throws NoSuchElementException;      E element();
175    
176      /**      /**
177       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue,
178       * 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.  
179       *       *
180       * @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
181       */       */
182      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;  
183  }  }

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