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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 : dl 1.17 * Whatever the ordering used, the <em>head</em> of the queue is that
20 :     * element which would be removed by a call to {@link #remove() } or
21 :     * {@link #poll()}. In a FIFO queue, all new elements are inserted at
22 :     * the <em> tail</em> of the queue. Other kinds of queues may use
23 :     * different placement rules. Every <tt>Queue</tt> implementation
24 :     * must specify its ordering properties.
25 :     *
26 :     * <p>The {@link #offer offer} method inserts an element if possible,
27 :     * otherwise returning <tt>false</tt>. This differs from the {@link
28 :     * java.util.Collection#add Collection.add} method, which can fail to
29 :     * add an element only by throwing an unchecked exception. The
30 :     * <tt>offer</tt> method is designed for use when failure is a normal,
31 :     * rather than exceptional occurrence, for example, in fixed-capacity
32 :     * (or &quot;bounded&quot;) queues.
33 : tim 1.9 *
34 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
35 : dholmes 1.8 * return the head of the queue.
36 :     * Exactly which element is removed from the queue is a
37 : dl 1.7 * function of the queue's ordering policy, which differs from
38 : dholmes 1.8 * implementation to implementation. The <tt>remove()</tt> and
39 : dl 1.7 * <tt>poll()</tt> methods differ only in their behavior when the
40 :     * queue is empty: the <tt>remove()</tt> method throws an exception,
41 :     * while the <tt>poll()</tt> method returns <tt>null</tt>.
42 : tim 1.1 *
43 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
44 : dholmes 1.10 * not remove, the head of the queue.
45 : tim 1.1 *
46 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
47 :     * methods</i>, which are common in concurrent programming. These methods,
48 :     * which wait for elements to appear or for space to become available, are
49 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
50 :     * extends this interface.
51 :     *
52 : dl 1.7 * <p><tt>Queue</tt> implementations generally do not allow insertion
53 :     * of <tt>null</tt> elements, although some implementations, such as
54 : brian 1.6 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
55 : dl 1.7 * Even in the implementations that permit it, <tt>null</tt> should
56 :     * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
57 :     * used as a special return value by the <tt>poll</tt> method to
58 :     * indicate that the queue contains no elements.
59 : tim 1.2 *
60 : dl 1.16 * <p><tt>Queue</tt> implementations generally do not define
61 :     * element-based versions of methods <tt>equals</tt> and
62 :     * <tt>hashCode</tt> but instead inherit the identity based versions
63 :     * from class <tt>Object</tt>, because element-based equality is not
64 :     * always well-defined for queues with the same elements but different
65 :     * ordering properties.
66 :     *
67 :     *
68 : tim 1.2 * <p>This interface is a member of the
69 :     * <a href="{@docRoot}/../guide/collections/index.html">
70 :     * Java Collections Framework</a>.
71 :     *
72 : tim 1.11 * @see java.util.Collection
73 : tim 1.2 * @see LinkedList
74 :     * @see PriorityQueue
75 : dholmes 1.15 * @see java.util.concurrent.LinkedBlockingQueue
76 : tim 1.2 * @see java.util.concurrent.BlockingQueue
77 :     * @see java.util.concurrent.ArrayBlockingQueue
78 :     * @see java.util.concurrent.LinkedBlockingQueue
79 :     * @see java.util.concurrent.PriorityBlockingQueue
80 : dl 1.7 * @since 1.5
81 :     * @author Doug Lea
82 : tim 1.2 */
83 : tim 1.1 public interface Queue<E> extends Collection<E> {
84 : dholmes 1.8
85 : tim 1.1 /**
86 : dl 1.17 * Inserts the specified element to this queue, if possible. When
87 :     * using queues that may impose insertion restrictions (for
88 :     * example capacity bounds), method <tt>offer</tt> is generally
89 :     * preferable to method {@link Collection#add}, which can fail to
90 :     * insert an element only by throwing an exception.
91 : tim 1.2 *
92 : dl 1.17 * @param o the element to insert.
93 : tim 1.9 * @return <tt>true</tt> if it was possible to add the element to
94 : dholmes 1.10 * this queue, else <tt>false</tt>
95 : tim 1.2 */
96 : dholmes 1.10 boolean offer(E o);
97 : tim 1.1
98 :     /**
99 : dl 1.18 * Retrieves and removes the head of this queue, or <tt>null</tt>
100 :     * if this queue is empty.
101 : tim 1.2 *
102 : dholmes 1.8 * @return the head of this queue, or <tt>null</tt> if this
103 : tim 1.9 * queue is empty.
104 : tim 1.2 */
105 : dl 1.7 E poll();
106 : tim 1.1
107 :     /**
108 : dl 1.18 * Retrieves and removes the head of this queue. This method
109 :     * differs from the <tt>poll</tt> method in that it throws an
110 :     * exception if this queue is empty.
111 : tim 1.2 *
112 : dholmes 1.8 * @return the head of this queue.
113 :     * @throws NoSuchElementException if this queue is empty.
114 : tim 1.2 */
115 : tim 1.9 E remove();
116 : tim 1.1
117 :     /**
118 : dl 1.18 * Retrieves, but does not remove, the head of this queue,
119 :     * returning <tt>null</tt> if this queue is empty.
120 : tim 1.2 *
121 : dl 1.18 * @return the head of this queue, or <tt>null</tt> if this queue
122 :     * is empty.
123 : tim 1.2 */
124 : dl 1.7 E peek();
125 : tim 1.1
126 :     /**
127 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
128 : tim 1.9 * differs from the <tt>peek</tt> method only in that it throws an
129 : dholmes 1.8 * exception if this queue is empty.
130 : tim 1.2 *
131 : dholmes 1.8 * @return the head of this queue.
132 :     * @throws NoSuchElementException if this queue is empty.
133 : tim 1.2 */
134 : tim 1.9 E element();
135 : tim 1.1 }

Doug Lea
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