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1 : dl 1.3 /*
2 :     * Written by Doug Lea with assistance from members of JCP JSR-166
3 :     * Expert Group and released to the public domain. Use, modify, and
4 :     * redistribute this code in any way without acknowledgement.
5 :     */
6 :    
7 : tim 1.1 package java.util;
8 :    
9 :     /**
10 : dholmes 1.8 * A collection designed for holding elements prior to processing.
11 : tim 1.11 * Besides basic {@link java.util.Collection Collection} operations, queues provide
12 : tim 1.2 * additional insertion, extraction, and inspection operations.
13 : dholmes 1.8 *
14 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
15 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
16 :     * priority queues, which order elements according to a supplied
17 : dholmes 1.8 * comparator, or the elements' natural ordering, and LIFO queues (or
18 :     * stacks) which order the elements LIFO (last-in-first-out).
19 :     * Whatever the ordering used, the <em>head</em> of the queue is that element
20 :     * which would be removed by a call to {@link #remove() } or {@link #poll()}.
21 : dholmes 1.10 * Every <tt>Queue</tt> implementation must specify its ordering guarantees.
22 : tim 1.2 *
23 : dholmes 1.13 * <p>The {@link #offer offer} method adds an element if possible, otherwise
24 :     * returning <tt>false</tt>. This differs from the
25 :     * {@link java.util.Collection#add Collection.add}
26 : tim 1.11 * method, which throws an unchecked exception upon
27 : tim 1.1 * failure. It is designed for use in collections in which failure to
28 :     * add is a normal, rather than exceptional occurrence, for example,
29 : dholmes 1.8 * in fixed-capacity (or &quot;bounded&quot;) queues.
30 : tim 1.9 *
31 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
32 : dholmes 1.8 * return the head of the queue.
33 :     * Exactly which element is removed from the queue is a
34 : dl 1.7 * function of the queue's ordering policy, which differs from
35 : dholmes 1.8 * implementation to implementation. The <tt>remove()</tt> and
36 : dl 1.7 * <tt>poll()</tt> methods differ only in their behavior when the
37 :     * queue is empty: the <tt>remove()</tt> method throws an exception,
38 :     * while the <tt>poll()</tt> method returns <tt>null</tt>.
39 : tim 1.1 *
40 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
41 : dholmes 1.10 * not remove, the head of the queue.
42 : tim 1.1 *
43 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
44 :     * methods</i>, which are common in concurrent programming. These methods,
45 :     * which wait for elements to appear or for space to become available, are
46 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
47 :     * extends this interface.
48 :     *
49 : dl 1.7 * <p><tt>Queue</tt> implementations generally do not allow insertion
50 :     * of <tt>null</tt> elements, although some implementations, such as
51 : brian 1.6 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
52 : dl 1.7 * Even in the implementations that permit it, <tt>null</tt> should
53 :     * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
54 :     * used as a special return value by the <tt>poll</tt> method to
55 :     * indicate that the queue contains no elements.
56 : tim 1.2 *
57 :     * <p>This interface is a member of the
58 :     * <a href="{@docRoot}/../guide/collections/index.html">
59 :     * Java Collections Framework</a>.
60 :     *
61 : tim 1.11 * @see java.util.Collection
62 : tim 1.2 * @see LinkedList
63 :     * @see PriorityQueue
64 : brian 1.6 * @see java.util.concurrent.LinkedQueue
65 : tim 1.2 * @see java.util.concurrent.BlockingQueue
66 :     * @see java.util.concurrent.ArrayBlockingQueue
67 :     * @see java.util.concurrent.LinkedBlockingQueue
68 :     * @see java.util.concurrent.PriorityBlockingQueue
69 : dl 1.7 * @since 1.5
70 :     * @author Doug Lea
71 : tim 1.2 */
72 : tim 1.1 public interface Queue<E> extends Collection<E> {
73 : dholmes 1.8
74 : tim 1.1 /**
75 : dholmes 1.14 * Adds the specified element to this queue, if possible.
76 : tim 1.2 *
77 : dholmes 1.10 * @param o the element to add.
78 : tim 1.9 * @return <tt>true</tt> if it was possible to add the element to
79 : dholmes 1.10 * this queue, else <tt>false</tt>
80 : tim 1.2 */
81 : dholmes 1.10 boolean offer(E o);
82 : tim 1.1
83 :     /**
84 : dholmes 1.14 * Retrieves and removes the head of this queue, if it is available.
85 : tim 1.2 *
86 : dholmes 1.8 * @return the head of this queue, or <tt>null</tt> if this
87 : tim 1.9 * queue is empty.
88 : tim 1.2 */
89 : dl 1.7 E poll();
90 : tim 1.1
91 :     /**
92 : dholmes 1.14 * Retrieves and removes the head of this queue.
93 : dholmes 1.8 * This method differs
94 : dholmes 1.10 * from the <tt>poll</tt> method in that it throws an exception if this
95 : tim 1.9 * queue is empty.
96 : tim 1.2 *
97 : dholmes 1.8 * @return the head of this queue.
98 :     * @throws NoSuchElementException if this queue is empty.
99 : tim 1.2 */
100 : tim 1.9 E remove();
101 : tim 1.1
102 :     /**
103 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue.
104 : dholmes 1.10 * This method differs from the <tt>poll</tt>
105 :     * method only in that this method does not remove the head element from
106 : dholmes 1.8 * this queue.
107 : tim 1.2 *
108 : dholmes 1.8 * @return the head of this queue, or <tt>null</tt> if this queue is empty.
109 : tim 1.2 */
110 : dl 1.7 E peek();
111 : tim 1.1
112 :     /**
113 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
114 : tim 1.9 * differs from the <tt>peek</tt> method only in that it throws an
115 : dholmes 1.8 * exception if this queue is empty.
116 : tim 1.2 *
117 : dholmes 1.8 * @return the head of this queue.
118 :     * @throws NoSuchElementException if this queue is empty.
119 : tim 1.2 */
120 : tim 1.9 E element();
121 : tim 1.1 }
122 : dholmes 1.8
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Doug Lea
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