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revision 1.1, Wed May 14 21:30:45 2003 UTC revision 1.29, Sat May 14 02:11:52 2005 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/licenses/publicdomain
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 java.util.Collection Collection} operations,
12   *   * queues provide additional insertion, extraction, and inspection
13   * <p> Queues typically, but do not necessarily order elements in a   * operations.  Each of these methods exists in two forms: one throws
14   * FIFO (first-in-first-out) manner. Among the exceptions are priority   * an exception if the operation fails, the other returns a special
15   * queues, that order elements in accord with supplied   * value (either <tt>null</tt> or <tt>false</tt>, depending on the
16   * Comparators. Every Queue implementation must specify its ordering   * operation).  The latter form of the insert operation is designed
17   * guarantees,   * specifically for use with capacity-restricted <tt>Queue</tt>
18   *   * implementations; in most implementations, insert operations cannot
19   * <p> The <tt>offer</tt> method adds an element if possible,   * fail.
20   * otherwise returning <tt>false</tt>. This differs from the   *
21   * Collections.add method, that throws an unchecked exception upon   * <p>
22   * failure. It is designed for use in collections in which failure to   * <table BORDER CELLPADDING=3 CELLSPACING=1>
23   * add is a normal, rather than exceptional occurrence, for example,   *  <tr>
24   * in fixed-capacity queues.   *    <td></td>
25   *   *    <td ALIGN=CENTER><em>Throws exception</em></td>
26   * <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return   *    <td ALIGN=CENTER><em>Special value</em></td>
27   * an element in accord with the implementation's ordering policies --   *  </tr>
28   * for example, in FIFO queues, it will return the oldest element.   *  <tr>
29   * The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior   *    <td><b>Insert</b></td>
30   * when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while   *    <td>{@link #add add(e)}</td>
31   * <tt>remove</tt> throws an exception. These are designed for usage   *    <td>{@link #offer offer(e)}</td>
32   * contexts in which emptiness is considered to be normal versus   *  </tr>
33   * exceptional.   *  <tr>
34   *   *    <td><b>Remove</b></td>
35   * <p> The <tt>element</tt> and <tt>peek</tt> methods return but do   *    <td>{@link #remove remove()}</td>
36   * not delete the element that would be obtained by a call to   *    <td>{@link #poll poll()}</td>
37   * <tt>remove</tt> and <tt>poll</tt> respectively.   *  </tr>
38   *   *  <tr>
39   * <p> The Queue interface does not define blocking queue methods   *    <td><b>Examine</b></td>
40   * (i.e., those that wait for elements to appear and/or for space to   *    <td>{@link #element element()}</td>
41   * be available) that are common in concurrent programming. These are   *    <td>{@link #peek peek()}</td>
42   * defined in the extended java.util.concurrent.BlockingQueue   *  </tr>
43   * interface.   * </table>
44   *   *
45   * <p> Queue implementations generally do not allow insertion of   * <p>Queues typically, but do not necessarily, order elements in a
46   * <tt>null</tt>. Even in those that allow it, it is a very bad idea   * FIFO (first-in-first-out) manner.  Among the exceptions are
47   * to do so, since <tt>null</tt> is also used as a sentinel by   * priority queues, which order elements according to a supplied
48   * <tt>poll</tt> to indicate that no elements exist.   * comparator, or the elements' natural ordering, and LIFO queues (or
49   **/   * 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     * element which would be removed by a call to {@link #remove() } or
52     * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
53     * the <em> tail</em> of the queue. Other kinds of queues may use
54     * different placement rules.  Every <tt>Queue</tt> implementation
55     * must specify its ordering properties.
56     *
57     * <p>The {@link #offer offer} method inserts an element if possible,
58     * otherwise returning <tt>false</tt>.  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     * <tt>offer</tt> 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
66     * return the head of the queue.
67     * Exactly which element is removed from the queue is a
68     * function of the queue's ordering policy, which differs from
69     * implementation to implementation. The <tt>remove()</tt> and
70     * <tt>poll()</tt> methods differ only in their behavior when the
71     * queue is empty: the <tt>remove()</tt> method throws an exception,
72     * while the <tt>poll()</tt> method returns <tt>null</tt>.
73     *
74     * <p>The {@link #element()} and {@link #peek()} methods return, but do
75     * not remove, the head of the queue.
76     *
77     * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
78     * methods</i>, which are common in concurrent programming.  These methods,
79     * which wait for elements to appear or for space to become available, are
80     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
81     * extends this interface.
82     *
83     * <p><tt>Queue</tt> implementations generally do not allow insertion
84     * of <tt>null</tt> elements, although some implementations, such as
85     * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
86     * Even in the implementations that permit it, <tt>null</tt> should
87     * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
88     * used as a special return value by the <tt>poll</tt> method to
89     * indicate that the queue contains no elements.
90     *
91     * <p><tt>Queue</tt> implementations generally do not define
92     * element-based versions of methods <tt>equals</tt> and
93     * <tt>hashCode</tt> but instead inherit the identity based versions
94     * from class <tt>Object</tt>, because element-based equality is not
95     * always well-defined for queues with the same elements but different
96     * ordering properties.
97     *
98     *
99     * <p>This interface is a member of the
100     * <a href="{@docRoot}/../guide/collections/index.html">
101     * Java Collections Framework</a>.
102     *
103     * @see java.util.Collection
104     * @see LinkedList
105     * @see PriorityQueue
106     * @see java.util.concurrent.LinkedBlockingQueue
107     * @see java.util.concurrent.BlockingQueue
108     * @see java.util.concurrent.ArrayBlockingQueue
109     * @see java.util.concurrent.LinkedBlockingQueue
110     * @see java.util.concurrent.PriorityBlockingQueue
111     * @since 1.5
112     * @author Doug Lea
113     * @param <E> the type of elements held in this collection
114     */
115  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
   
116      /**      /**
117       * Add the given object to this queue if possible.       * Inserts the specified element into this queue if it is possible to do so
118       * @param x the object to add       * immediately without violating capacity restrictions, returning
119       * @return true if successful       * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
120       **/       * if no space is currently available.
121      public boolean offer(E x);       *
122         * @param e the element to add
123      /**       * @return <tt>true</tt> (as per the spec for {@link Collection#add})
124       * Delete and return an object from the queue if one is available.       * @throws IllegalStateException if the element cannot be added at this
125       * @return the object, or null if the queue is empty.       *         time due to capacity restrictions
126       **/       * @throws NullPointerException if the specified element is null and
127      public E poll();       *         this queue not permit null elements
128         * @throws ClassCastException if the class of the specified element
129      /**       *         prevents it from being added to this queue
130       * Delete and return the element produced by poll, if the queue is       * @throws IllegalArgumentException if some property of this element
131       * not empty.       *         prevents it from being added to this queue
132       * @return an element       */
133       * @throws NoSuchElementException if empty      boolean add(E e);
134       **/  
135      public E remove() throws NoSuchElementException;      /**
136         * Inserts the specified element into this queue if it is possible to do
137      /**       * so immediately without violating capacity restrictions.
138       * Return but do not delete the element that will be returned by       * When using a capacity-restricted deque, this method is generally
139       * the next call to poll.       * preferable to {@link #add}, which can fail to insert an element only
140       * @return an element, or null if empty       * by throwing an exception.
141       **/       *
142      public E peek();       * @param e the element to add
143         * @return <tt>true</tt> if the element was added to this queue, else
144      /**       *         <tt>false</tt>
145       * Return but do not delete the element that will be returned by       * @throws NullPointerException if the specified element is null and
146       * the next call to poll, if the queue is not empty.       *         this queue does not permit null elements
147       * @return an element       * @throws ClassCastException if the class of the specified element
148       * @throws NoSuchElementException if empty       *         prevents it from being added to this queue
149       **/       * @throws IllegalArgumentException if some property of this element
150      public E element() throws NoSuchElementException;       *         prevents it from being added to this queue
151         */
152        boolean offer(E e);
153    
154        /**
155         * Retrieves and removes the head of this queue.  This method differs
156         * from {@link #poll} only in that it throws an exception if this queue
157         * is empty.
158         *
159         * @return the head of this queue
160         * @throws NoSuchElementException if this queue is empty
161         */
162        E remove();
163    
164        /**
165         * Retrieves and removes the head of this queue,
166         * or returns <tt>null</tt> if this queue is empty.
167         *
168         * @return the head of this queue, or <tt>null</tt> if this queue is empty
169         */
170        E poll();
171    
172        /**
173         * Retrieves, but does not remove, the head of this queue.  This method
174         * differs from {@link #peek} only in that it throws an exception if
175         * this queue is empty.
176         *
177         * @return the head of this queue
178         * @throws NoSuchElementException if this queue is empty
179         */
180        E element();
181    
182        /**
183         * Retrieves, but does not remove, the head of this queue,
184         * or returns <tt>null</tt> if this queue is empty.
185         *
186         * @return the head of this queue, or <tt>null</tt> if this queue is empty
187         */
188        E peek();
189  }  }

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