/[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 1.3 - (show annotations)
Tue May 27 18:20:06 2003 UTC (16 years, 2 months ago) by dl
Branch: MAIN
CVS Tags: JSR166_PRERELEASE_0_1
Changes since 1.2: +9 -3 lines
re-checkin initial implementations

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 priority
16 * queues, which order elements according to a supplied comparators, or
17 * the elements natural ordering. Every Queue implementation must specify
18 * 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 * <p>The {@link #remove()} and {@link #poll()} methods remove and return an
28 * element in accord with the implementation's ordering policy. For example,
29 * in FIFO queues, they remove and return the oldest element in the queue.
30 * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their
31 * behavior when the queue is empty: the <tt>remove()</tt> method throws an
32 * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
33 *
34 * <p>The {@link #element()} and {@link #peek()} methods return but do
35 * not delete the element that would be obtained by a call to
36 * the <tt>remove</tt> and <tt>poll</tt> methods respectively.
37 *
38 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
39 * methods</i>, which are common in concurrent programming. These methods,
40 * which wait for elements to appear or for space to become available, are
41 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
42 * extends this interface.
43 *
44 * <p><tt>Queue</tt> implementations generally do not allow insertion of
45 * <tt>null</tt> elements. Even in the few implementations that permit it,
46 * it is a bad idea, as <tt>null</tt> is also used as a special return value
47 * by the <tt>poll</tt> method to indicate that the queue contains no
48 * elements.
49 *
50 * <p>This interface is a member of the
51 * <a href="{@docRoot}/../guide/collections/index.html">
52 * Java Collections Framework</a>.
53 *
54 * @see Collection
55 * @see LinkedList
56 * @see PriorityQueue
57 * @see LinkedQueue
58 * @see java.util.concurrent.BlockingQueue
59 * @see java.util.concurrent.ArrayBlockingQueue
60 * @see java.util.concurrent.LinkedBlockingQueue
61 * @see java.util.concurrent.PriorityBlockingQueue
62 */
63 public interface Queue<E> extends Collection<E> {
64 /**
65 * Add the specified element to this queue, if possible.
66 *
67 * @param element the element to add.
68 * @return true if it was possible to add the element to the queue.
69 */
70 public boolean offer(E element);
71
72 /**
73 * Remove and return an element from the queue if one is available.
74 * Exactly which element is removed from the queue is a function
75 * of the queue's ordering policy, which differs from implementation
76 * to implementation. Possible orderings include (but are not limited
77 * to) first-in-first-out (FIFO), element priority, and arbitrary.
78 *
79 * @return an element previously on the queue, or <tt>null</tt> if the
80 * queue is empty.
81 */
82 public E poll();
83
84 /**
85 * Remove and return an element from the queue. This method differs
86 * from the <tt>poll</tt> method in that it throws an exception if the
87 * queue is empty.
88 *
89 * @return an element previously on the queue.
90 * @throws NoSuchElementException if the queue is empty.
91 */
92 public E remove() throws NoSuchElementException;
93
94 /**
95 * Return, but do not remove, an element from the queue, or <tt>null</tt>
96 * if the queue is empty. This method returns the same object reference
97 * that would be returned by by the <tt>poll</tt> method. The two methods
98 * differ in that this method does not remove the element from the queue.
99 *
100 * @return an element on the queue, or <tt>null</tt> if the queue is empty.
101 */
102 public E peek();
103
104 /**
105 * Return, but do not remove, an element from the queue. This method
106 * differs from the <tt>peek</tt> method in that it throws an exception if
107 * the queue is empty.
108 *
109 * @return an element on the queue.
110 * @throws NoSuchElementException if the queue is empty.
111 */
112 public E element() throws NoSuchElementException;
113 }

dl@cs.oswego.edu
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