/[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 1.16 - (show annotations)
Sat Aug 30 11:40:04 2003 UTC (15 years, 11 months ago) by dl
Branch: MAIN
Changes since 1.15: +8 -0 lines
Improve PQ.remove; mention that Queues don't usually define equals, hashCode

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 java.util.Collection 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 offer} method adds an element if possible, otherwise
24 * returning <tt>false</tt>. This differs from the
25 * {@link java.util.Collection#add Collection.add}
26 * method, which throws an unchecked exception upon
27 * failure. It is designed for use in collections in which failure to
28 * add is a normal, rather than exceptional occurrence, for example,
29 * in fixed-capacity (or &quot;bounded&quot;) queues.
30 *
31 * <p>The {@link #remove()} and {@link #poll()} methods remove and
32 * return the head of the queue.
33 * Exactly which element is removed from the queue is a
34 * function of the queue's ordering policy, which differs from
35 * implementation to implementation. The <tt>remove()</tt> and
36 * <tt>poll()</tt> methods differ only in their behavior when the
37 * queue is empty: the <tt>remove()</tt> method throws an exception,
38 * while the <tt>poll()</tt> method returns <tt>null</tt>.
39 *
40 * <p>The {@link #element()} and {@link #peek()} methods return, but do
41 * not remove, the head of the queue.
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><tt>Queue</tt> implementations generally do not define
58 * element-based versions of methods <tt>equals</tt> and
59 * <tt>hashCode</tt> but instead inherit the identity based versions
60 * from class <tt>Object</tt>, because element-based equality is not
61 * always well-defined for queues with the same elements but different
62 * ordering properties.
63 *
64 *
65 * <p>This interface is a member of the
66 * <a href="{@docRoot}/../guide/collections/index.html">
67 * Java Collections Framework</a>.
68 *
69 * @see java.util.Collection
70 * @see LinkedList
71 * @see PriorityQueue
72 * @see java.util.concurrent.LinkedBlockingQueue
73 * @see java.util.concurrent.BlockingQueue
74 * @see java.util.concurrent.ArrayBlockingQueue
75 * @see java.util.concurrent.LinkedBlockingQueue
76 * @see java.util.concurrent.PriorityBlockingQueue
77 * @since 1.5
78 * @author Doug Lea
79 */
80 public interface Queue<E> extends Collection<E> {
81
82 /**
83 * Adds the specified element to this queue, if possible.
84 *
85 * @param o the element to add.
86 * @return <tt>true</tt> if it was possible to add the element to
87 * this queue, else <tt>false</tt>
88 */
89 boolean offer(E o);
90
91 /**
92 * Retrieves and removes the head of this queue, if it is available.
93 *
94 * @return the head of this queue, or <tt>null</tt> if this
95 * queue is empty.
96 */
97 E poll();
98
99 /**
100 * Retrieves and removes the head of this queue.
101 * This method differs
102 * from the <tt>poll</tt> method in that it throws an exception if this
103 * queue is empty.
104 *
105 * @return the head of this queue.
106 * @throws NoSuchElementException if this queue is empty.
107 */
108 E remove();
109
110 /**
111 * Retrieves, but does not remove, the head of this queue.
112 * This method differs from the <tt>poll</tt>
113 * method only in that this method does not remove the head element from
114 * this queue.
115 *
116 * @return the head of this queue, or <tt>null</tt> if this queue is empty.
117 */
118 E peek();
119
120 /**
121 * Retrieves, but does not remove, the head of this queue. This method
122 * differs from the <tt>peek</tt> method only in that it throws an
123 * exception if this queue is empty.
124 *
125 * @return the head of this queue.
126 * @throws NoSuchElementException if this queue is empty.
127 */
128 E element();
129 }
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dl@cs.oswego.edu
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