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revision 1.1, Wed May 14 21:30:45 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
3     * Expert Group and released to the public domain, as explained at
4     * http://creativecommons.org/publicdomain/zero/1.0/
5     */
6    
7  package java.util;  package java.util;
8    
9  /**  /**
10   * Queues are Collections supporting additional basic insertion,   * A collection designed for holding elements prior to processing.
11   * extraction, and inspection operations.   * Besides basic {@link Collection} operations, queues provide
12   *   * additional insertion, extraction, and inspection operations.
13   * <p> Queues typically, but do not necessarily order elements in a   * Each of these methods exists in two forms: one throws an exception
14   * FIFO (first-in-first-out) manner. Among the exceptions are priority   * if the operation fails, the other returns a special value (either
15   * queues, that order elements in accord with supplied   * {@code null} or {@code false}, depending on the operation).  The
16   * Comparators. Every Queue implementation must specify its ordering   * latter form of the insert operation is designed specifically for
17   * guarantees,   * use with capacity-restricted {@code Queue} implementations; in most
18   *   * implementations, insert operations cannot fail.
19   * <p> The <tt>offer</tt> method adds an element if possible,   *
20   * otherwise returning <tt>false</tt>. This differs from the   * <table BORDER CELLPADDING=3 CELLSPACING=1>
21   * Collections.add method, that throws an unchecked exception upon   * <caption>Summary of Queue methods</caption>
22   * failure. It is designed for use in collections in which failure to   *  <tr>
23   * add is a normal, rather than exceptional occurrence, for example,   *    <td></td>
24   * in fixed-capacity queues.   *    <td style="text-align:center"><em>Throws exception</em></td>
25   *   *    <td style="text-align:center"><em>Returns special value</em></td>
26   * <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return   *  </tr>
27   * an element in accord with the implementation's ordering policies --   *  <tr>
28   * for example, in FIFO queues, it will return the oldest element.   *    <td><b>Insert</b></td>
29   * The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior   *    <td>{@link #add(Object) add(e)}</td>
30   * when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while   *    <td>{@link #offer(Object) offer(e)}</td>
31   * <tt>remove</tt> throws an exception. These are designed for usage   *  </tr>
32   * contexts in which emptiness is considered to be normal versus   *  <tr>
33   * exceptional.   *    <td><b>Remove</b></td>
34   *   *    <td>{@link #remove() remove()}</td>
35   * <p> The <tt>element</tt> and <tt>peek</tt> methods return but do   *    <td>{@link #poll() poll()}</td>
36   * not delete the element that would be obtained by a call to   *  </tr>
37   * <tt>remove</tt> and <tt>poll</tt> respectively.   *  <tr>
38   *   *    <td><b>Examine</b></td>
39   * <p> The Queue interface does not define blocking queue methods   *    <td>{@link #element() element()}</td>
40   * (i.e., those that wait for elements to appear and/or for space to   *    <td>{@link #peek() peek()}</td>
41   * be available) that are common in concurrent programming. These are   *  </tr>
42   * defined in the extended java.util.concurrent.BlockingQueue   * </table>
43   * interface.   *
44   *   * <p>Queues typically, but do not necessarily, order elements in a
45   * <p> Queue implementations generally do not allow insertion of   * FIFO (first-in-first-out) manner.  Among the exceptions are
46   * <tt>null</tt>. Even in those that allow it, it is a very bad idea   * priority queues, which order elements according to a supplied
47   * to do so, since <tt>null</tt> is also used as a sentinel by   * comparator, or the elements' natural ordering, and LIFO queues (or
48   * <tt>poll</tt> to indicate that no elements exist.   * 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     * 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
52     * the <em>tail</em> of the queue. Other kinds of queues may use
53     * different placement rules.  Every {@code Queue} implementation
54     * must specify its ordering properties.
55     *
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
65     * return the head of the queue.
66     * Exactly which element is removed from the queue is a
67     * function of the queue's ordering policy, which differs from
68     * implementation to implementation. The {@code remove()} and
69     * {@code poll()} methods differ only in their behavior when the
70     * queue is empty: the {@code remove()} method throws an exception,
71     * while the {@code poll()} method returns {@code null}.
72     *
73     * <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,
78     * which wait for elements to appear or for space to become available, are
79     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
80     * extends this interface.
81     *
82     * <p>{@code Queue} implementations generally do not allow insertion
83     * of {@code null} elements, although some implementations, such as
84     * {@link LinkedList}, do not prohibit insertion of {@code null}.
85     * Even in the implementations that permit it, {@code null} should
86     * not be inserted into a {@code Queue}, as {@code null} is also
87     * used as a special return value by the {@code poll} method to
88     * 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
98     * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
99     * Java Collections Framework</a>.
100     *
101     * @since 1.5
102     * @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 given object to this queue if possible.       * Inserts the specified element into this queue if it is possible to do so
108       * @param x the object to add       * immediately without violating capacity restrictions, returning
109       * @return true if successful       * {@code true} upon success and throwing an {@code IllegalStateException}
110       **/       * if no space is currently available.
111      public boolean offer(E x);       *
112         * @param e the element to add
113      /**       * @return {@code true} (as specified by {@link Collection#add})
114       * Delete and return an object from the queue if one is available.       * @throws IllegalStateException if the element cannot be added at this
115       * @return the object, or null if the queue is empty.       *         time due to capacity restrictions
116       **/       * @throws ClassCastException if the class of the specified element
117      public E poll();       *         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       * Delete and return the element produced by poll, if the queue is       * @throws IllegalArgumentException if some property of this element
121       * not empty.       *         prevents it from being added to this queue
122       * @return an element       */
123       * @throws NoSuchElementException if empty      boolean add(E e);
124       **/  
125      public E remove() throws NoSuchElementException;      /**
126         * Inserts the specified element into this queue if it is possible to do
127      /**       * so immediately without violating capacity restrictions.
128       * Return but do not delete the element that will be returned by       * When using a capacity-restricted queue, this method is generally
129       * the next call to poll.       * preferable to {@link #add}, which can fail to insert an element only
130       * @return an element, or null if empty       * by throwing an exception.
131       **/       *
132      public E peek();       * @param e the element to add
133         * @return {@code true} if the element was added to this queue, else
134      /**       *         {@code false}
135       * Return but do not delete the element that will be returned by       * @throws ClassCastException if the class of the specified element
136       * the next call to poll, if the queue is not empty.       *         prevents it from being added to this queue
137       * @return an element       * @throws NullPointerException if the specified element is null and
138       * @throws NoSuchElementException if empty       *         this queue does not permit null elements
139       **/       * @throws IllegalArgumentException if some property of this element
140      public E element() throws NoSuchElementException;       *         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        E remove();
153    
154        /**
155         * Retrieves and removes the head of this queue,
156         * or returns {@code null} if this queue is empty.
157         *
158         * @return the head of this queue, or {@code null} if this queue is empty
159         */
160        E poll();
161    
162        /**
163         * Retrieves, but does not remove, the head of this queue.  This method
164         * differs from {@link #peek peek} only in that it throws an exception
165         * if this queue is empty.
166         *
167         * @return the head of this queue
168         * @throws NoSuchElementException if this queue is empty
169         */
170        E element();
171    
172        /**
173         * Retrieves, but does not remove, the head of this queue,
174         * or returns {@code null} if this queue is empty.
175         *
176         * @return the head of this queue, or {@code null} if this queue is empty
177         */
178        E peek();
179  }  }

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