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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 1.15 by dholmes, Wed Aug 6 01:57:53 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 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 queues.
29	>	* in fixed-capacity (or &quot;bounded&quot;) queues.
30		*
31	<	* <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return
32	<	* an element in accord with the implementation's ordering policies --
33	<	* for example, in FIFO queues, it will return the oldest element.
34	<	* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior
35	<	* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while
36	<	* <tt>remove</tt> throws an exception. These are designed for usage
37	<	* contexts in which emptiness is considered to be normal versus
38	<	* exceptional.
39	<	*
40	<	* <p> The <tt>element</tt> and <tt>peek</tt> methods return but do
41	<	* not delete the element that would be obtained by a call to
42	<	* <tt>remove</tt> and <tt>poll</tt> respectively.
43	<	*
44	<	* <p> The Queue interface does not define blocking queue methods
45	<	* (i.e., those that wait for elements to appear and/or for space to
46	<	* be available) that are common in concurrent programming. These are
47	<	* defined in the extended java.util.concurrent.BlockingQueue
48	<	* interface.
49	<	*
50	<	* <p> Queue implementations generally do not allow insertion of
51	<	* <tt>null</tt>. Even in those that allow it, it is a very bad idea
52	<	* to do so, since <tt>null</tt> is also used as a sentinel by
53	<	* <tt>poll</tt> to indicate that no elements exist.
54	<	**/
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>This interface is a member of the
58	>	* <a href="{@docRoot}/../guide/collections/index.html">
59	>	* Java Collections Framework</a>.
60	>	*
61	>	* @see java.util.Collection
62	>	* @see LinkedList
63	>	* @see PriorityQueue
64	>	* @see java.util.concurrent.LinkedBlockingQueue
65	>	* @see java.util.concurrent.BlockingQueue
66	>	* @see java.util.concurrent.ArrayBlockingQueue
67	>	* @see java.util.concurrent.LinkedBlockingQueue
68	>	* @see java.util.concurrent.PriorityBlockingQueue
69	>	* @since 1.5
70	>	* @author Doug Lea
71	>	*/
72		public interface Queue<E> extends Collection<E> {
73
74		/**
75	<	* Add the given object to this queue if possible.
76	<	* @param x the object to add
77	<	* @return true if successful
78	<	**/
79	<	public boolean offer(E x);
75	>	* Adds the specified element to this queue, if possible.
76	>	*
77	>	* @param o the element to add.
78	>	* @return <tt>true</tt> if it was possible to add the element to
79	>	* this queue, else <tt>false</tt>
80	>	*/
81	>	boolean offer(E o);
82
83		/**
84	<	* Delete and return an object from the queue if one is available.
85	<	* @return the object, or null if the queue is empty.
86	<	**/
87	<	public E poll();
84	>	* Retrieves and removes the head of this queue, if it is available.
85	>	*
86	>	* @return the head of this queue, or <tt>null</tt> if this
87	>	*         queue is empty.
88	>	*/
89	>	E poll();
90
91		/**
92	<	* Delete and return the element produced by poll, if the queue is
93	<	* not empty.
94	<	* @return an element
95	<	* @throws NoSuchElementException if empty
96	<	**/
97	<	public E remove() throws NoSuchElementException;
92	>	* Retrieves and removes the head of this queue.
93	>	* This method differs
94	>	* from the <tt>poll</tt> method in that it throws an exception if this
95	>	* queue is empty.
96	>	*
97	>	* @return the head of this queue.
98	>	* @throws NoSuchElementException if this queue is empty.
99	>	*/
100	>	E remove();
101
102		/**
103	<	* Return but do not delete the element that will be returned by
104	<	* the next call to poll.
105	<	* @return an element, or null if empty
106	<	**/
107	<	public E peek();
103	>	* Retrieves, but does not remove, the head of this queue.
104	>	* This method differs from the <tt>poll</tt>
105	>	* method only in that this method does not remove the head element from
106	>	* this queue.
107	>	*
108	>	* @return the head of this queue, or <tt>null</tt> if this queue is empty.
109	>	*/
110	>	E peek();
111
112		/**
113	<	* Return but do not delete the element that will be returned by
114	<	* the next call to poll, if the queue is not empty.
115	<	* @return an element
116	<	* @throws NoSuchElementException if empty
117	<	**/
118	<	public E element() throws NoSuchElementException;
113	>	* Retrieves, but does not remove, the head of this queue.  This method
114	>	* differs from the <tt>peek</tt> method only in that it throws an
115	>	* exception if this queue is empty.
116	>	*
117	>	* @return the head of this queue.
118	>	* @throws NoSuchElementException if this queue is empty.
119	>	*/
120	>	E element();
121		}
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