/[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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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 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 {@code null} or {@code false}, depending on the
16 * operation). The latter form of the insert operation is designed
17 * specifically for use with capacity-restricted {@code Queue}
18 * implementations; in most implementations, insert operations cannot
19 * fail.
20 *
21 * <table BORDER CELLPADDING=3 CELLSPACING=1>
22 * <caption>Summary of Queue methods</caption>
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 Queue#add add(e)}</td>
31 * <td>{@link Queue#offer offer(e)}</td>
32 * </tr>
33 * <tr>
34 * <td><b>Remove</b></td>
35 * <td>{@link Queue#remove remove()}</td>
36 * <td>{@link Queue#poll poll()}</td>
37 * </tr>
38 * <tr>
39 * <td><b>Examine</b></td>
40 * <td>{@link Queue#element element()}</td>
41 * <td>{@link Queue#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 {@code Queue} implementation
55 * must specify its ordering properties.
56 *
57 * <p>The {@link #offer offer} method inserts an element if possible,
58 * otherwise returning {@code false}. 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 * {@code offer} 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 {@code remove()} and
70 * {@code poll()} methods differ only in their behavior when the
71 * queue is empty: the {@code remove()} method throws an exception,
72 * while the {@code poll()} method returns {@code null}.
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 {@code Queue} 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>{@code Queue} implementations generally do not allow insertion
84 * of {@code null} elements, although some implementations, such as
85 * {@link LinkedList}, do not prohibit insertion of {@code null}.
86 * Even in the implementations that permit it, {@code null} should
87 * not be inserted into a {@code Queue}, as {@code null} is also
88 * used as a special return value by the {@code poll} method to
89 * indicate that the queue contains no elements.
90 *
91 * <p>{@code Queue} implementations generally do not define
92 * element-based versions of methods {@code equals} and
93 * {@code hashCode} but instead inherit the identity based versions
94 * from class {@code Object}, because element-based equality is not
95 * always well-defined for queues with the same elements but different
96 * ordering properties.
97 *
98 * <p>This interface is a member of the
99 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
100 * Java Collections Framework</a>.
101 *
102 * @since 1.5
103 * @author Doug Lea
104 * @param <E> the type of elements held in this queue
105 */
106 public interface Queue<E> extends Collection<E> {
107 /**
108 * Inserts the specified element into this queue if it is possible to do so
109 * immediately without violating capacity restrictions, returning
110 * {@code true} upon success and throwing an {@code IllegalStateException}
111 * if no space is currently available.
112 *
113 * @param e the element to add
114 * @return {@code true} (as specified by {@link Collection#add})
115 * @throws IllegalStateException if the element cannot be added at this
116 * time due to capacity restrictions
117 * @throws ClassCastException if the class of the specified element
118 * prevents it from being added to this queue
119 * @throws NullPointerException if the specified element is null and
120 * this queue does not permit null elements
121 * @throws IllegalArgumentException if some property of this element
122 * prevents it from being added to this queue
123 */
124 boolean add(E e);
125
126 /**
127 * Inserts the specified element into this queue if it is possible to do
128 * so immediately without violating capacity restrictions.
129 * When using a capacity-restricted queue, this method is generally
130 * preferable to {@link #add}, which can fail to insert an element only
131 * by throwing an exception.
132 *
133 * @param e the element to add
134 * @return {@code true} if the element was added to this queue, else
135 * {@code false}
136 * @throws ClassCastException if the class of the specified element
137 * prevents it from being added to this queue
138 * @throws NullPointerException if the specified element is null and
139 * this queue does not permit null elements
140 * @throws IllegalArgumentException if some property of this element
141 * prevents it from being added to this queue
142 */
143 boolean offer(E e);
144
145 /**
146 * Retrieves and removes the head of this queue. This method differs
147 * from {@link #poll poll} only in that it throws an exception if this
148 * queue is empty.
149 *
150 * @return the head of this queue
151 * @throws NoSuchElementException if this queue is empty
152 */
153 E remove();
154
155 /**
156 * Retrieves and removes the head of this queue,
157 * or returns {@code null} if this queue is empty.
158 *
159 * @return the head of this queue, or {@code null} if this queue is empty
160 */
161 E poll();
162
163 /**
164 * Retrieves, but does not remove, the head of this queue. This method
165 * differs from {@link #peek peek} only in that it throws an exception
166 * if this queue is empty.
167 *
168 * @return the head of this queue
169 * @throws NoSuchElementException if this queue is empty
170 */
171 E element();
172
173 /**
174 * Retrieves, but does not remove, the head of this queue,
175 * or returns {@code null} if this queue is empty.
176 *
177 * @return the head of this queue, or {@code null} if this queue is empty
178 */
179 E peek();
180 }

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