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root/jsr166/jsr166/src/main/java/util/Queue.java

Comparing jsr166/src/main/java/util/Queue.java (file contents):
Revision 1.1 by tim, Wed May 14 21:30:45 2003 UTC vs.
Revision 1.6 by brian, Mon Jun 23 02:26:15 2003 UTC

#	Line 1 | Line 1
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	<	* Queues are Collections supporting additional basic insertion,
11	<	* extraction, and inspection operations.
12	<	*
13	<	* <p> Queues typically, but do not necessarily order elements in a
14	<	* FIFO (first-in-first-out) manner. Among the exceptions are priority
15	<	* queues, that order elements in accord with supplied
16	<	* Comparators. Every Queue implementation must specify its ordering
17	<	* guarantees,
18	<	*
19	<	* <p> The <tt>offer</tt> method adds an element if possible,
20	<	* otherwise returning <tt>false</tt>. This differs from the
21	<	* Collections.add method, that throws an unchecked exception upon
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 queues.
26	<	*
27	<	* <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return
28	<	* an element in accord with the implementation's ordering policies --
29	<	* for example, in FIFO queues, it will return the oldest element.
30	<	* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior
31	<	* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while
32	<	* <tt>remove</tt> throws an exception. These are designed for usage
33	<	* contexts in which emptiness is considered to be normal versus
34	<	* exceptional.
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 <tt>element</tt> and <tt>peek</tt> methods return but do
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	<	* <tt>remove</tt> and <tt>poll</tt> respectively.
40	>	* the <tt>remove</tt> and <tt>poll</tt> methods respectively.
41		*
42	<	* <p> The Queue interface does not define blocking queue methods
43	<	* (i.e., those that wait for elements to appear and/or for space to
44	<	* be available) that are common in concurrent programming. These are
45	<	* defined in the extended java.util.concurrent.BlockingQueue
46	<	* interface.
47	<	*
48	<	* <p> Queue implementations generally do not allow insertion of
49	<	* <tt>null</tt>. Even in those that allow it, it is a very bad idea
50	<	* to do so, since <tt>null</tt> is also used as a sentinel by
51	<	* <tt>poll</tt> to indicate that no elements exist.
52	<	**/
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> {
45	–
70		/**
71	<	* Add the given object to this queue if possible.
72	<	* @param x the object to add
73	<	* @return true if successful
74	<	**/
75	<	public boolean offer(E x);
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	<	* Delete and return an object from the queue if one is available.
80	<	* @return the object, or null if the queue is empty.
81	<	**/
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	<	* Delete and return the element produced by poll, if the queue is
88	<	* not empty.
89	<	* @return an element
90	<	* @throws NoSuchElementException if empty
91	<	**/
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 delete the element that will be returned by
98	<	* the next call to poll.
99	<	* @return an element, or null if empty
100	<	**/
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 delete the element that will be returned by
108	<	* the next call to poll, if the queue is not empty.
109	<	* @return an element
110	<	* @throws NoSuchElementException if empty
111	<	**/
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		}

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