/[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 1.10 - (show annotations)
Thu Jul 31 07:18:02 2003 UTC (16 years ago) by dholmes
Branch: MAIN
Changes since 1.9: +9 -9 lines
Continued updates to explicit and inherited doc comments.
Consistency over remove(null)
Some inherited doc is still not right.

1 /*
2 * Written by Doug Lea with assistance from members of JCP JSR-166
3 * Expert Group and released to the public domain. Use, modify, and
4 * redistribute this code in any way without acknowledgement.
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 *
14 * <p>Queues typically, but do not necessarily, order elements in a
15 * FIFO (first-in-first-out) manner. Among the exceptions are
16 * priority queues, which order elements according to a supplied
17 * comparator, or the elements' natural ordering, and LIFO queues (or
18 * stacks) which order the elements LIFO (last-in-first-out).
19 * Whatever the ordering used, the <em>head</em> of the queue is that element
20 * which would be removed by a call to {@link #remove() } or {@link #poll()}.
21 * Every <tt>Queue</tt> implementation must specify its ordering guarantees.
22 *
23 * <p>The {@link #offer(E)} method adds an element if possible, otherwise
24 * returning <tt>false</tt>. This differs from the {@link
25 * Collections#add(Object)} method, which throws an unchecked exception upon
26 * failure. It is designed for use in collections in which failure to
27 * add is a normal, rather than exceptional occurrence, for example,
28 * in fixed-capacity (or &quot;bounded&quot;) queues.
29 *
30 * <p>The {@link #remove()} and {@link #poll()} methods remove and
31 * return the head of the queue.
32 * Exactly which element is removed from the queue is a
33 * function of the queue's ordering policy, which differs from
34 * implementation to implementation. The <tt>remove()</tt> and
35 * <tt>poll()</tt> methods differ only in their behavior when the
36 * queue is empty: the <tt>remove()</tt> method throws an exception,
37 * while the <tt>poll()</tt> method returns <tt>null</tt>.
38 *
39 * <p>The {@link #element()} and {@link #peek()} methods return, but do
40 * not remove, the head of the queue.
41 *
42 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
43 * methods</i>, which are common in concurrent programming. These methods,
44 * which wait for elements to appear or for space to become available, are
45 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
46 * extends this interface.
47 *
48 * <p><tt>Queue</tt> implementations generally do not allow insertion
49 * of <tt>null</tt> elements, although some implementations, such as
50 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
51 * Even in the implementations that permit it, <tt>null</tt> should
52 * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
53 * used as a special return value by the <tt>poll</tt> method to
54 * indicate that the queue contains no elements.
55 *
56 * <p>This interface is a member of the
57 * <a href="{@docRoot}/../guide/collections/index.html">
58 * Java Collections Framework</a>.
59 *
60 * @see Collection
61 * @see LinkedList
62 * @see PriorityQueue
63 * @see java.util.concurrent.LinkedQueue
64 * @see java.util.concurrent.BlockingQueue
65 * @see java.util.concurrent.ArrayBlockingQueue
66 * @see java.util.concurrent.LinkedBlockingQueue
67 * @see java.util.concurrent.PriorityBlockingQueue
68 * @since 1.5
69 * @author Doug Lea
70 */
71 public interface Queue<E> extends Collection<E> {
72
73 /**
74 * Add the specified element to this queue, if possible.
75 *
76 * @param o the element to add.
77 * @return <tt>true</tt> if it was possible to add the element to
78 * this queue, else <tt>false</tt>
79 */
80 boolean offer(E o);
81
82 /**
83 * Retrieve and remove the head of this queue, if it is available.
84 *
85 * @return the head of this queue, or <tt>null</tt> if this
86 * queue is empty.
87 */
88 E poll();
89
90 /**
91 * Retrieve and remove the head of this queue.
92 * This method differs
93 * from the <tt>poll</tt> method in that it throws an exception if this
94 * queue is empty.
95 *
96 * @return the head of this queue.
97 * @throws NoSuchElementException if this queue is empty.
98 */
99 E remove();
100
101 /**
102 * Retrieve, but do not remove, the head of this queue.
103 * This method differs from the <tt>poll</tt>
104 * method only in that this method does not remove the head element from
105 * this queue.
106 *
107 * @return the head of this queue, or <tt>null</tt> if this queue is empty.
108 */
109 E peek();
110
111 /**
112 * Retrieve, but do not remove, the head of this queue. This method
113 * differs from the <tt>peek</tt> method only in that it throws an
114 * exception if this queue is empty.
115 *
116 * @return the head of this queue.
117 * @throws NoSuchElementException if this queue is empty.
118 */
119 E element();
120 }
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dl@cs.oswego.edu
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