/[cvs]/jsr166/src/main/java/util/Queue.java
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Contents of /jsr166/src/main/java/util/Queue.java

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Revision 1.23 - (show annotations)
Sun Nov 21 01:40:39 2004 UTC (14 years, 9 months ago) by dl
Branch: MAIN
Changes since 1.22: +25 -1 lines
Changes for maintenance/RFE phase

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;
8
9 /**
10 * A collection designed for holding elements prior to processing.
11 * Besides basic {@link java.util.Collection Collection} operations, queues provide
12 * 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 *
38 * <p>Queues typically, but do not necessarily, order elements in a
39 * FIFO (first-in-first-out) manner. Among the exceptions are
40 * priority queues, which order elements according to a supplied
41 * comparator, or the elements' natural ordering, and LIFO queues (or
42 * stacks) which order the elements LIFO (last-in-first-out).
43 * 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 *
58 * <p>The {@link #remove()} and {@link #poll()} methods remove and
59 * return the head of the queue.
60 * Exactly which element is removed from the queue is a
61 * function of the queue's ordering policy, which differs from
62 * implementation to implementation. The <tt>remove()</tt> and
63 * <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 *
67 * <p>The {@link #element()} and {@link #peek()} methods return, but do
68 * not remove, the head of the queue.
69 *
70 * <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 * <p><tt>Queue</tt> implementations generally do not allow insertion
77 * of <tt>null</tt> elements, although some implementations, such as
78 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
79 * 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 *
84 * <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 * <p>This interface is a member of the
93 * <a href="{@docRoot}/../guide/collections/index.html">
94 * Java Collections Framework</a>.
95 *
96 * @see java.util.Collection
97 * @see LinkedList
98 * @see PriorityQueue
99 * @see java.util.concurrent.LinkedBlockingQueue
100 * @see java.util.concurrent.BlockingQueue
101 * @see java.util.concurrent.ArrayBlockingQueue
102 * @see java.util.concurrent.LinkedBlockingQueue
103 * @see java.util.concurrent.PriorityBlockingQueue
104 * @since 1.5
105 * @author Doug Lea
106 * @param <E> the type of elements held in this collection
107 */
108 public interface Queue<E> extends Collection<E> {
109
110 /**
111 * Inserts the specified element into this queue, if possible. When
112 * 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 *
117 * @param o the element to insert.
118 * @return <tt>true</tt> if it was possible to add the element to
119 * this queue, else <tt>false</tt>
120 */
121 boolean offer(E o);
122
123 /**
124 * Retrieves and removes the head of this queue, or <tt>null</tt>
125 * if this queue is empty.
126 *
127 * @return the head of this queue, or <tt>null</tt> if this
128 * queue is empty.
129 */
130 E poll();
131
132 /**
133 * 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 *
137 * @return the head of this queue.
138 * @throws NoSuchElementException if this queue is empty.
139 */
140 E remove();
141
142 /**
143 * Retrieves, but does not remove, the head of this queue,
144 * returning <tt>null</tt> if this queue is empty.
145 *
146 * @return the head of this queue, or <tt>null</tt> if this queue
147 * is empty.
148 */
149 E peek();
150
151 /**
152 * Retrieves, but does not remove, the head of this queue. This method
153 * differs from the <tt>peek</tt> method only in that it throws an
154 * exception if this queue is empty.
155 *
156 * @return the head of this queue.
157 * @throws NoSuchElementException if this queue is empty.
158 */
159 E element();
160 }

dl@cs.oswego.edu
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