/[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 1.7 - (show annotations)
Tue Jun 24 14:34:30 2003 UTC (16 years, 2 months ago) by dl
Branch: MAIN
CVS Tags: JSR166_PRELIMINARY_TEST_RELEASE_2
Changes since 1.6: +23 -20 lines
Added missing javadoc tags; minor reformatting

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 0 *
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. Every Queue
18 * implementation must specify its ordering guarantees.
19 *
20 * <p>The {@link #offer(E)} method adds an element if possible, otherwise
21 * returning <tt>false</tt>. This differs from the {@link
22 * Collections#add(Object)} method, which throws an unchecked exception upon
23 * failure. It is designed for use in collections in which failure to
24 * add is a normal, rather than exceptional occurrence, for example,
25 * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.
26
27 *
28 * <p>The {@link #remove()} and {@link #poll()} methods remove and
29 * return an element in accord with the implementation's ordering
30 * policy. Exactly which element is removed from the queue is a
31 * function of the queue's ordering policy, which differs from
32 * implementation to implementation. Possible orderings include (but
33 * are not limited to) first-in-first-out (FIFO), last-in-first-out
34 * (LIFO), element priority, and arbitrary. 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 delete the element that would be obtained by a call to
41 * the <tt>remove</tt> and <tt>poll</tt> methods respectively.
42 *
43 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
44 * methods</i>, which are common in concurrent programming. These methods,
45 * which wait for elements to appear or for space to become available, are
46 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
47 * extends this interface.
48 *
49 * <p><tt>Queue</tt> implementations generally do not allow insertion
50 * of <tt>null</tt> elements, although some implementations, such as
51 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
52 * Even in the implementations that permit it, <tt>null</tt> should
53 * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
54 * used as a special return value by the <tt>poll</tt> method to
55 * indicate that the queue contains no elements.
56 *
57 * <p>This interface is a member of the
58 * <a href="{@docRoot}/../guide/collections/index.html">
59 * Java Collections Framework</a>.
60 *
61 * @see Collection
62 * @see LinkedList
63 * @see PriorityQueue
64 * @see java.util.concurrent.LinkedQueue
65 * @see java.util.concurrent.BlockingQueue
66 * @see java.util.concurrent.ArrayBlockingQueue
67 * @see java.util.concurrent.LinkedBlockingQueue
68 * @see java.util.concurrent.PriorityBlockingQueue
69 * @since 1.5
70 * @author Doug Lea
71 */
72 public interface Queue<E> extends Collection<E> {
73 /**
74 * Add the specified element to this queue, if possible.
75 *
76 * @param element the element to add.
77 * @return true if it was possible to add the element to the queue.
78 */
79 boolean offer(E element);
80
81 /**
82 * Remove and return an element from the queue if one is available.
83 *
84 * @return an element previously on the queue, or <tt>null</tt> if the
85 * queue is empty.
86 */
87 E poll();
88
89 /**
90 * Remove and return an element from the queue. This method differs
91 * from the <tt>poll</tt> method in that it throws an exception if the
92 * queue is empty.
93 *
94 * @return an element previously on the queue.
95 * @throws NoSuchElementException if the queue is empty.
96 */
97 E remove() throws NoSuchElementException;
98
99 /**
100 * Return, but do not remove, an element from the queue, or <tt>null</tt>
101 * if the queue is empty. This method returns the same object reference
102 * that would be returned by by the <tt>poll</tt> method. The two methods
103 * differ in that this method does not remove the element from the queue.
104 *
105 * @return an element on the queue, or <tt>null</tt> if the queue is empty.
106 */
107 E peek();
108
109 /**
110 * Return, but do not remove, an element from the queue. This method
111 * differs from the <tt>peek</tt> method in that it throws an exception if
112 * the queue is empty.
113 *
114 * @return an element on the queue.
115 * @throws NoSuchElementException if the queue is empty.
116 */
117 E element() throws NoSuchElementException;
118 }

dl@cs.oswego.edu
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