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1 : dl 1.3 /*
2 :     * Written by Doug Lea with assistance from members of JCP JSR-166
3 : dl 1.22 * Expert Group and released to the public domain, as explained at
4 :     * http://creativecommons.org/licenses/publicdomain
5 : dl 1.3 */
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 : dl 1.23 * additional insertion, extraction, and inspection operations. The names
13 :     * of the operations vary with their policies:
14 :     *
15 :     *<table BORDER CELLPADDING=3 CELLSPACING=1>
16 :     * <tr>
17 :     * <td><em>Operation</em></td>
18 :     * <td ALIGN=CENTER><em>Attempt</em></td>
19 :     * <td ALIGN=CENTER><em>Throw</em></td>
20 :     * </tr>
21 :     * <tr>
22 :     * <td><em>insert</em></td>
23 :     * <td>offer(x)</td>
24 :     * <td>add(x)</td>
25 :     * </tr>
26 :     * <tr>
27 :     * <td><em>extract</em></td>
28 :     * <td>poll()</td>
29 :     * <td>remove()</td>
30 :     * </tr>
31 :     * <tr>
32 :     * <td><em>inspect</em></td>
33 :     * <td>peek()</td>
34 :     * <td>element()</td>
35 :     * </tr>
36 :     *</table>
37 : dholmes 1.8 *
38 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
39 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
40 :     * priority queues, which order elements according to a supplied
41 : dholmes 1.8 * comparator, or the elements' natural ordering, and LIFO queues (or
42 :     * stacks) which order the elements LIFO (last-in-first-out).
43 : dl 1.17 * Whatever the ordering used, the <em>head</em> of the queue is that
44 :     * element which would be removed by a call to {@link #remove() } or
45 :     * {@link #poll()}. In a FIFO queue, all new elements are inserted at
46 :     * the <em> tail</em> of the queue. Other kinds of queues may use
47 :     * different placement rules. Every <tt>Queue</tt> implementation
48 :     * must specify its ordering properties.
49 :     *
50 :     * <p>The {@link #offer offer} method inserts an element if possible,
51 :     * otherwise returning <tt>false</tt>. This differs from the {@link
52 :     * java.util.Collection#add Collection.add} method, which can fail to
53 :     * add an element only by throwing an unchecked exception. The
54 :     * <tt>offer</tt> method is designed for use when failure is a normal,
55 :     * rather than exceptional occurrence, for example, in fixed-capacity
56 :     * (or &quot;bounded&quot;) queues.
57 : tim 1.9 *
58 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
59 : dholmes 1.8 * return the head of the queue.
60 :     * Exactly which element is removed from the queue is a
61 : dl 1.7 * function of the queue's ordering policy, which differs from
62 : dholmes 1.8 * implementation to implementation. The <tt>remove()</tt> and
63 : dl 1.7 * <tt>poll()</tt> methods differ only in their behavior when the
64 :     * queue is empty: the <tt>remove()</tt> method throws an exception,
65 :     * while the <tt>poll()</tt> method returns <tt>null</tt>.
66 : tim 1.1 *
67 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
68 : dholmes 1.10 * not remove, the head of the queue.
69 : tim 1.1 *
70 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
71 :     * methods</i>, which are common in concurrent programming. These methods,
72 :     * which wait for elements to appear or for space to become available, are
73 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
74 :     * extends this interface.
75 :     *
76 : dl 1.7 * <p><tt>Queue</tt> implementations generally do not allow insertion
77 :     * of <tt>null</tt> elements, although some implementations, such as
78 : brian 1.6 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
79 : dl 1.7 * Even in the implementations that permit it, <tt>null</tt> should
80 :     * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
81 :     * used as a special return value by the <tt>poll</tt> method to
82 :     * indicate that the queue contains no elements.
83 : tim 1.2 *
84 : dl 1.16 * <p><tt>Queue</tt> implementations generally do not define
85 :     * element-based versions of methods <tt>equals</tt> and
86 :     * <tt>hashCode</tt> but instead inherit the identity based versions
87 :     * from class <tt>Object</tt>, because element-based equality is not
88 :     * always well-defined for queues with the same elements but different
89 :     * ordering properties.
90 :     *
91 :     *
92 : tim 1.2 * <p>This interface is a member of the
93 :     * <a href="{@docRoot}/../guide/collections/index.html">
94 :     * Java Collections Framework</a>.
95 :     *
96 : tim 1.11 * @see java.util.Collection
97 : tim 1.2 * @see LinkedList
98 :     * @see PriorityQueue
99 : dholmes 1.15 * @see java.util.concurrent.LinkedBlockingQueue
100 : tim 1.2 * @see java.util.concurrent.BlockingQueue
101 :     * @see java.util.concurrent.ArrayBlockingQueue
102 :     * @see java.util.concurrent.LinkedBlockingQueue
103 :     * @see java.util.concurrent.PriorityBlockingQueue
104 : dl 1.7 * @since 1.5
105 :     * @author Doug Lea
106 : dl 1.21 * @param <E> the type of elements held in this collection
107 : tim 1.2 */
108 : tim 1.1 public interface Queue<E> extends Collection<E> {
109 : dholmes 1.8
110 : tim 1.1 /**
111 : dl 1.19 * Inserts the specified element into this queue, if possible. When
112 : dl 1.17 * using queues that may impose insertion restrictions (for
113 :     * example capacity bounds), method <tt>offer</tt> is generally
114 :     * preferable to method {@link Collection#add}, which can fail to
115 :     * insert an element only by throwing an exception.
116 : tim 1.2 *
117 : dl 1.17 * @param o the element to insert.
118 : tim 1.9 * @return <tt>true</tt> if it was possible to add the element to
119 : dholmes 1.10 * this queue, else <tt>false</tt>
120 : tim 1.2 */
121 : dholmes 1.10 boolean offer(E o);
122 : tim 1.1
123 :     /**
124 : dl 1.18 * Retrieves and removes the head of this queue, or <tt>null</tt>
125 :     * if this queue is empty.
126 : tim 1.2 *
127 : dholmes 1.8 * @return the head of this queue, or <tt>null</tt> if this
128 : tim 1.9 * queue is empty.
129 : tim 1.2 */
130 : dl 1.7 E poll();
131 : tim 1.1
132 :     /**
133 : dl 1.18 * Retrieves and removes the head of this queue. This method
134 :     * differs from the <tt>poll</tt> method in that it throws an
135 :     * exception if this queue is empty.
136 : tim 1.2 *
137 : dholmes 1.8 * @return the head of this queue.
138 :     * @throws NoSuchElementException if this queue is empty.
139 : tim 1.2 */
140 : tim 1.9 E remove();
141 : tim 1.1
142 :     /**
143 : dl 1.18 * Retrieves, but does not remove, the head of this queue,
144 :     * returning <tt>null</tt> if this queue is empty.
145 : tim 1.2 *
146 : dl 1.18 * @return the head of this queue, or <tt>null</tt> if this queue
147 :     * is empty.
148 : tim 1.2 */
149 : dl 1.7 E peek();
150 : tim 1.1
151 :     /**
152 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
153 : tim 1.9 * differs from the <tt>peek</tt> method only in that it throws an
154 : dholmes 1.8 * exception if this queue is empty.
155 : tim 1.2 *
156 : dholmes 1.8 * @return the head of this queue.
157 :     * @throws NoSuchElementException if this queue is empty.
158 : tim 1.2 */
159 : tim 1.9 E element();
160 : tim 1.1 }

Doug Lea
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