/[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 1.28 - (show annotations)
Mon May 2 17:34:02 2005 UTC (14 years, 3 months ago) by jsr166
Branch: MAIN
Changes since 1.27: +2 -1 lines
<p> before <table>

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,
12 * queues provide additional insertion, extraction, and inspection
13 * operations. Each of these methods exists in two forms: one throws
14 * an exception if the operation fails, the other returns a special
15 * value (either <tt>null</tt> or <tt>false</tt>, depending on the
16 * operation). The latter form of the insert operation is designed
17 * specifically for use with capacity-restricted <tt>Queue</tt>
18 * implementations; in most implementations, insert operations cannot
19 * fail.
20 *
21 * <p>
22 * <table BORDER CELLPADDING=3 CELLSPACING=1>
23 * <tr>
24 * <td></td>
25 * <td ALIGN=CENTER><em>Throws exception</em></td>
26 * <td ALIGN=CENTER><em>Returns special value</em></td>
27 * </tr>
28 * <tr>
29 * <td><b>Insert</b></td>
30 * <td>{@link #add add(e)}</td>
31 * <td>{@link #offer offer(e)}</td>
32 * </tr>
33 * <tr>
34 * <td><b>Remove</b></td>
35 * <td>{@link #remove remove()}</td>
36 * <td>{@link #poll poll()}</td>
37 * </tr>
38 * <tr>
39 * <td><b>Examine</b></td>
40 * <td>{@link #element element()}</td>
41 * <td>{@link #peek peek()}</td>
42 * </tr>
43 * </table>
44 *
45 * <p>Queues typically, but do not necessarily, order elements in a
46 * FIFO (first-in-first-out) manner. Among the exceptions are
47 * priority queues, which order elements according to a supplied
48 * 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> {
116
117 /**
118 * Inserts the specified element into this queue, if possible. When
119 * using queues that may impose insertion restrictions (for
120 * example capacity bounds), method <tt>offer</tt> is generally
121 * preferable to method {@link Collection#add}, which can fail to
122 * insert an element only by throwing an exception.
123 *
124 * @param e the element to insert.
125 * @return <tt>true</tt> if it was possible to add the element to
126 * this queue, else <tt>false</tt>
127 */
128 boolean offer(E e);
129
130 /**
131 * Retrieves and removes the head of this queue, or <tt>null</tt>
132 * if this queue is empty.
133 *
134 * @return the head of this queue, or <tt>null</tt> if this
135 * queue is empty.
136 */
137 E poll();
138
139 /**
140 * Retrieves and removes the head of this queue. This method
141 * differs from the {@link #poll} method only in that it throws an
142 * exception if this queue is empty.
143 *
144 * @return the head of this queue.
145 * @throws NoSuchElementException if this queue is empty.
146 */
147 E remove();
148
149 /**
150 * Retrieves, but does not remove, the head of this queue,
151 * returning <tt>null</tt> if this queue is empty.
152 *
153 * @return the head of this queue, or <tt>null</tt> if this queue
154 * is empty.
155 */
156 E peek();
157
158 /**
159 * Retrieves, but does not remove, the head of this queue. This method
160 * differs from the {@link #peek} method only in that it throws an
161 * exception if this queue is empty.
162 *
163 * @return the head of this queue.
164 * @throws NoSuchElementException if this queue is empty.
165 */
166 E element();
167 }

dl@cs.oswego.edu
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