/[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 1.47 - (show annotations)
Sat May 6 06:55:50 2017 UTC (2 years, 3 months ago) by jsr166
Branch: MAIN
Changes since 1.46: +2 -2 lines
8179367: update use of align, valign attributes in java.base to use style attribute

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/publicdomain/zero/1.0/
5 */
6
7 package java.util;
8
9 /**
10 * A collection designed for holding elements prior to processing.
11 * Besides basic {@link Collection} operations, queues provide
12 * additional insertion, extraction, and inspection operations.
13 * Each of these methods exists in two forms: one throws an exception
14 * if the operation fails, the other returns a special value (either
15 * {@code null} or {@code false}, depending on the operation). The
16 * latter form of the insert operation is designed specifically for
17 * use with capacity-restricted {@code Queue} implementations; in most
18 * implementations, insert operations cannot fail.
19 *
20 * <table BORDER CELLPADDING=3 CELLSPACING=1>
21 * <caption>Summary of Queue methods</caption>
22 * <tr>
23 * <td></td>
24 * <td style="text-align:center"><em>Throws exception</em></td>
25 * <td style="text-align:center"><em>Returns special value</em></td>
26 * </tr>
27 * <tr>
28 * <td><b>Insert</b></td>
29 * <td>{@link #add(Object) add(e)}</td>
30 * <td>{@link #offer(Object) 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 {@code Queue} implementation
54 * must specify its ordering properties.
55 *
56 * <p>The {@link #offer offer} method inserts an element if possible,
57 * otherwise returning {@code false}. 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 * {@code offer} 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 {@code remove()} and
69 * {@code poll()} methods differ only in their behavior when the
70 * queue is empty: the {@code remove()} method throws an exception,
71 * while the {@code poll()} method returns {@code null}.
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 {@code Queue} 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>{@code Queue} implementations generally do not allow insertion
83 * of {@code null} elements, although some implementations, such as
84 * {@link LinkedList}, do not prohibit insertion of {@code null}.
85 * Even in the implementations that permit it, {@code null} should
86 * not be inserted into a {@code Queue}, as {@code null} is also
87 * used as a special return value by the {@code poll} method to
88 * indicate that the queue contains no elements.
89 *
90 * <p>{@code Queue} implementations generally do not define
91 * element-based versions of methods {@code equals} and
92 * {@code hashCode} but instead inherit the identity based versions
93 * from class {@code Object}, because element-based equality is not
94 * always well-defined for queues with the same elements but different
95 * ordering properties.
96 *
97 * <p>This interface is a member of the
98 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
99 * Java Collections Framework</a>.
100 *
101 * @since 1.5
102 * @author Doug Lea
103 * @param <E> the type of elements held in this queue
104 */
105 public interface Queue<E> extends Collection<E> {
106 /**
107 * Inserts the specified element into this queue if it is possible to do so
108 * immediately without violating capacity restrictions, returning
109 * {@code true} upon success and throwing an {@code IllegalStateException}
110 * if no space is currently available.
111 *
112 * @param e the element to add
113 * @return {@code true} (as specified by {@link Collection#add})
114 * @throws IllegalStateException if the element cannot be added at this
115 * time due to capacity restrictions
116 * @throws ClassCastException if the class of the specified element
117 * prevents it from being added to this queue
118 * @throws NullPointerException if the specified element is null and
119 * this queue does not permit null elements
120 * @throws IllegalArgumentException if some property of this element
121 * prevents it from being added to this queue
122 */
123 boolean add(E e);
124
125 /**
126 * Inserts the specified element into this queue if it is possible to do
127 * so immediately without violating capacity restrictions.
128 * When using a capacity-restricted queue, this method is generally
129 * preferable to {@link #add}, which can fail to insert an element only
130 * by throwing an exception.
131 *
132 * @param e the element to add
133 * @return {@code true} if the element was added to this queue, else
134 * {@code false}
135 * @throws ClassCastException if the class of the specified element
136 * prevents it from being added to this queue
137 * @throws NullPointerException if the specified element is null and
138 * this queue does not permit null elements
139 * @throws IllegalArgumentException if some property of this element
140 * prevents it from being added to this queue
141 */
142 boolean offer(E e);
143
144 /**
145 * Retrieves and removes the head of this queue. This method differs
146 * from {@link #poll() poll()} only in that it throws an exception if
147 * this queue is empty.
148 *
149 * @return the head of this queue
150 * @throws NoSuchElementException if this queue is empty
151 */
152 E remove();
153
154 /**
155 * Retrieves and removes the head of this queue,
156 * or returns {@code null} if this queue is empty.
157 *
158 * @return the head of this queue, or {@code null} if this queue is empty
159 */
160 E poll();
161
162 /**
163 * Retrieves, but does not remove, the head of this queue. This method
164 * differs from {@link #peek peek} only in that it throws an exception
165 * if this queue is empty.
166 *
167 * @return the head of this queue
168 * @throws NoSuchElementException if this queue is empty
169 */
170 E element();
171
172 /**
173 * Retrieves, but does not remove, the head of this queue,
174 * or returns {@code null} if this queue is empty.
175 *
176 * @return the head of this queue, or {@code null} if this queue is empty
177 */
178 E peek();
179 }

dl@cs.oswego.edu
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