/[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 1.27 - (show annotations)
Mon May 2 08:35:49 2005 UTC (14 years, 3 months ago) by jsr166
Branch: MAIN
Changes since 1.26: +2 -2 lines
E o -> E e

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><table BORDER CELLPADDING=3 CELLSPACING=1>
22 * <tr>
23 * <td></td>
24 * <td ALIGN=CENTER><em>Throws exception</em></td>
25 * <td ALIGN=CENTER><em>Returns special value</em></td>
26 * </tr>
27 * <tr>
28 * <td><b>Insert</b></td>
29 * <td>{@link #add add(e)}</td>
30 * <td>{@link #offer offer(e)}</td>
31 * </tr>
32 * <tr>
33 * <td><b>Remove</b></td>
34 * <td>{@link #remove remove()}</td>
35 * <td>{@link #poll poll()}</td>
36 * </tr>
37 * <tr>
38 * <td><b>Examine</b></td>
39 * <td>{@link #element element()}</td>
40 * <td>{@link #peek peek()}</td>
41 * </tr>
42 * </table>
43 *
44 * <p>Queues typically, but do not necessarily, order elements in a
45 * FIFO (first-in-first-out) manner. Among the exceptions are
46 * priority queues, which order elements according to a supplied
47 * comparator, or the elements' natural ordering, and LIFO queues (or
48 * stacks) which order the elements LIFO (last-in-first-out).
49 * Whatever the ordering used, the <em>head</em> of the queue is that
50 * element which would be removed by a call to {@link #remove() } or
51 * {@link #poll()}. In a FIFO queue, all new elements are inserted at
52 * the <em> tail</em> of the queue. Other kinds of queues may use
53 * different placement rules. Every <tt>Queue</tt> implementation
54 * must specify its ordering properties.
55 *
56 * <p>The {@link #offer offer} method inserts an element if possible,
57 * otherwise returning <tt>false</tt>. This differs from the {@link
58 * java.util.Collection#add Collection.add} method, which can fail to
59 * add an element only by throwing an unchecked exception. The
60 * <tt>offer</tt> method is designed for use when failure is a normal,
61 * rather than exceptional occurrence, for example, in fixed-capacity
62 * (or &quot;bounded&quot;) queues.
63 *
64 * <p>The {@link #remove()} and {@link #poll()} methods remove and
65 * return the head of the queue.
66 * Exactly which element is removed from the queue is a
67 * function of the queue's ordering policy, which differs from
68 * implementation to implementation. The <tt>remove()</tt> and
69 * <tt>poll()</tt> methods differ only in their behavior when the
70 * queue is empty: the <tt>remove()</tt> method throws an exception,
71 * while the <tt>poll()</tt> method returns <tt>null</tt>.
72 *
73 * <p>The {@link #element()} and {@link #peek()} methods return, but do
74 * not remove, the head of the queue.
75 *
76 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
77 * methods</i>, which are common in concurrent programming. These methods,
78 * which wait for elements to appear or for space to become available, are
79 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
80 * extends this interface.
81 *
82 * <p><tt>Queue</tt> implementations generally do not allow insertion
83 * of <tt>null</tt> elements, although some implementations, such as
84 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
85 * Even in the implementations that permit it, <tt>null</tt> should
86 * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
87 * used as a special return value by the <tt>poll</tt> method to
88 * indicate that the queue contains no elements.
89 *
90 * <p><tt>Queue</tt> implementations generally do not define
91 * element-based versions of methods <tt>equals</tt> and
92 * <tt>hashCode</tt> but instead inherit the identity based versions
93 * from class <tt>Object</tt>, because element-based equality is not
94 * always well-defined for queues with the same elements but different
95 * ordering properties.
96 *
97 *
98 * <p>This interface is a member of the
99 * <a href="{@docRoot}/../guide/collections/index.html">
100 * Java Collections Framework</a>.
101 *
102 * @see java.util.Collection
103 * @see LinkedList
104 * @see PriorityQueue
105 * @see java.util.concurrent.LinkedBlockingQueue
106 * @see java.util.concurrent.BlockingQueue
107 * @see java.util.concurrent.ArrayBlockingQueue
108 * @see java.util.concurrent.LinkedBlockingQueue
109 * @see java.util.concurrent.PriorityBlockingQueue
110 * @since 1.5
111 * @author Doug Lea
112 * @param <E> the type of elements held in this collection
113 */
114 public interface Queue<E> extends Collection<E> {
115
116 /**
117 * Inserts the specified element into this queue, if possible. When
118 * using queues that may impose insertion restrictions (for
119 * example capacity bounds), method <tt>offer</tt> is generally
120 * preferable to method {@link Collection#add}, which can fail to
121 * insert an element only by throwing an exception.
122 *
123 * @param e the element to insert.
124 * @return <tt>true</tt> if it was possible to add the element to
125 * this queue, else <tt>false</tt>
126 */
127 boolean offer(E e);
128
129 /**
130 * Retrieves and removes the head of this queue, or <tt>null</tt>
131 * if this queue is empty.
132 *
133 * @return the head of this queue, or <tt>null</tt> if this
134 * queue is empty.
135 */
136 E poll();
137
138 /**
139 * Retrieves and removes the head of this queue. This method
140 * differs from the {@link #poll} method only in that it throws an
141 * exception if this queue is empty.
142 *
143 * @return the head of this queue.
144 * @throws NoSuchElementException if this queue is empty.
145 */
146 E remove();
147
148 /**
149 * Retrieves, but does not remove, the head of this queue,
150 * returning <tt>null</tt> if this queue is empty.
151 *
152 * @return the head of this queue, or <tt>null</tt> if this queue
153 * is empty.
154 */
155 E peek();
156
157 /**
158 * Retrieves, but does not remove, the head of this queue. This method
159 * differs from the {@link #peek} method only in that it throws an
160 * exception if this queue is empty.
161 *
162 * @return the head of this queue.
163 * @throws NoSuchElementException if this queue is empty.
164 */
165 E element();
166 }

dl@cs.oswego.edu
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