/[cvs]/jsr166/src/main/java/util/Queue.java
ViewVC logotype

Contents of /jsr166/src/main/java/util/Queue.java

Parent Directory Parent Directory | Revision Log Revision Log


Revision 1.6 - (show annotations)
Mon Jun 23 02:26:15 2003 UTC (16 years, 2 months ago) by brian
Branch: MAIN
Changes since 1.5: +7 -5 lines
Partial javadoc pass

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 return an
29 * element in accord with the implementation's ordering policy.
30 * Exactly which element is removed from the queue is a function
31 * of the queue's ordering policy, which differs from implementation
32 * to implementation. Possible orderings include (but are not limited
33 * to) first-in-first-out (FIFO), last-in-first-out (LIFO), element priority, and arbitrary.
34 * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their
35 * behavior when the queue is empty: the <tt>remove()</tt> method throws an
36 * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
37 *
38 * <p>The {@link #element()} and {@link #peek()} methods return but do
39 * not delete the element that would be obtained by a call to
40 * the <tt>remove</tt> and <tt>poll</tt> methods respectively.
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 of
49 * <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 not be inserted into
52 * a <tt>Queue</tt>, as <tt>null</tt> is also used as a special return value
53 * by the <tt>poll</tt> method to indicate that the queue contains no
54 * 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 */
69 public interface Queue<E> extends Collection<E> {
70 /**
71 * Add the specified element to this queue, if possible.
72 *
73 * @param element the element to add.
74 * @return true if it was possible to add the element to the queue.
75 */
76 public boolean offer(E element);
77
78 /**
79 * Remove and return an element from the queue if one is available.
80 *
81 * @return an element previously on the queue, or <tt>null</tt> if the
82 * queue is empty.
83 */
84 public E poll();
85
86 /**
87 * Remove and return an element from the queue. This method differs
88 * from the <tt>poll</tt> method in that it throws an exception if the
89 * queue is empty.
90 *
91 * @return an element previously on the queue.
92 * @throws NoSuchElementException if the queue is empty.
93 */
94 public E remove() throws NoSuchElementException;
95
96 /**
97 * Return, but do not remove, an element from the queue, or <tt>null</tt>
98 * if the queue is empty. This method returns the same object reference
99 * that would be returned by by the <tt>poll</tt> method. The two methods
100 * differ in that this method does not remove the element from the queue.
101 *
102 * @return an element on the queue, or <tt>null</tt> if the queue is empty.
103 */
104 public E peek();
105
106 /**
107 * Return, but do not remove, an element from the queue. This method
108 * differs from the <tt>peek</tt> method in that it throws an exception if
109 * the queue is empty.
110 *
111 * @return an element on the queue.
112 * @throws NoSuchElementException if the queue is empty.
113 */
114 public E element() throws NoSuchElementException;
115 }

dl@cs.oswego.edu
ViewVC Help
Powered by ViewVC 1.1.27