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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 1.3 by dl, Tue May 27 18:20:06 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	>	*
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 queues.
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 <tt>remove</tt> and <tt>poll</tt> methods delete and return
21	<	* an element in accord with the implementation's ordering policies --
22	<	* for example, in FIFO queues, it will return the oldest element.
23	<	* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior
24	<	* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while
25	<	* <tt>remove</tt> throws an exception. These are designed for usage
26	<	* contexts in which emptiness is considered to be normal versus
27	<	* exceptional.
28	<	*
29	<	* <p> The <tt>element</tt> and <tt>peek</tt> methods return but do
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	<	* <tt>remove</tt> and <tt>poll</tt> respectively.
36	>	* the <tt>remove</tt> and <tt>poll</tt> methods respectively.
37		*
38	<	* <p> The Queue interface does not define blocking queue methods
39	<	* (i.e., those that wait for elements to appear and/or for space to
40	<	* be available) that are common in concurrent programming. These are
41	<	* defined in the extended java.util.concurrent.BlockingQueue
42	<	* interface.
43	<	*
44	<	* <p> Queue implementations generally do not allow insertion of
45	<	* <tt>null</tt>. Even in those that allow it, it is a very bad idea
46	<	* to do so, since <tt>null</tt> is also used as a sentinel by
47	<	* <tt>poll</tt> to indicate that no elements exist.
48	<	**/
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> {
45	–
64		/**
65	<	* Add the given object to this queue if possible.
66	<	* @param x the object to add
67	<	* @return true if successful
68	<	**/
69	<	public boolean offer(E x);
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	<	* Delete and return an object from the queue if one is available.
74	<	* @return the object, or null if the queue is empty.
75	<	**/
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	<	* Delete and return the element produced by poll, if the queue is
86	<	* not empty.
87	<	* @return an element
88	<	* @throws NoSuchElementException if empty
89	<	**/
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 delete the element that will be returned by
96	<	* the next call to poll.
97	<	* @return an element, or null if empty
98	<	**/
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 delete the element that will be returned by
106	<	* the next call to poll, if the queue is not empty.
107	<	* @return an element
108	<	* @throws NoSuchElementException if empty
109	<	**/
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		}

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