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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 1.22 by dl, Sat Dec 27 19:26:15 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, as explained at
4	+	* http://creativecommons.org/licenses/publicdomain
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
22	<	* failure. It is designed for use in collections in which failure to
23	<	* add is a normal, rather than exceptional occurrence, for example,
24	<	* in fixed-capacity queues.
25	<	*
26	<	* <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return
27	<	* an element in accord with the implementation's ordering policies --
28	<	* for example, in FIFO queues, it will return the oldest element.
29	<	* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior
30	<	* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while
31	<	* <tt>remove</tt> throws an exception. These are designed for usage
32	<	* contexts in which emptiness is considered to be normal versus
33	<	* exceptional.
34	<	*
35	<	* <p> The <tt>element</tt> and <tt>peek</tt> methods return but do
36	<	* not delete the element that would be obtained by a call to
37	<	* <tt>remove</tt> and <tt>poll</tt> respectively.
38	<	*
39	<	* <p> The Queue interface does not define blocking queue methods
40	<	* (i.e., those that wait for elements to appear and/or for space to
41	<	* be available) that are common in concurrent programming. These are
42	<	* defined in the extended java.util.concurrent.BlockingQueue
43	<	* interface.
44	<	*
45	<	* <p> Queue implementations generally do not allow insertion of
46	<	* <tt>null</tt>. Even in those that allow it, it is a very bad idea
47	<	* to do so, since <tt>null</tt> is also used as a sentinel by
48	<	* <tt>poll</tt> to indicate that no elements exist.
49	<	**/
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
20	>	* element which would be removed by a call to {@link #remove() } or
21	>	* {@link #poll()}.  In a FIFO queue, all new elements are inserted at
22	>	* the <em> tail</em> of the queue. Other kinds of queues may use
23	>	* different placement rules.  Every <tt>Queue</tt> implementation
24	>	* must specify its ordering properties.
25	>	*
26	>	* <p>The {@link #offer offer} method inserts an element if possible,
27	>	* otherwise returning <tt>false</tt>.  This differs from the {@link
28	>	* java.util.Collection#add Collection.add} method, which can fail to
29	>	* add an element only by throwing an unchecked exception.  The
30	>	* <tt>offer</tt> method is designed for use when failure is a normal,
31	>	* rather than exceptional occurrence, for example, in fixed-capacity
32	>	* (or &quot;bounded&quot;) queues.
33	>	*
34	>	* <p>The {@link #remove()} and {@link #poll()} methods remove and
35	>	* return the head of the queue.
36	>	* Exactly which element is removed from the queue is a
37	>	* function of the queue's ordering policy, which differs from
38	>	* implementation to implementation. The <tt>remove()</tt> and
39	>	* <tt>poll()</tt> methods differ only in their behavior when the
40	>	* queue is empty: the <tt>remove()</tt> method throws an exception,
41	>	* while the <tt>poll()</tt> method returns <tt>null</tt>.
42	>	*
43	>	* <p>The {@link #element()} and {@link #peek()} methods return, but do
44	>	* not remove, the head of the queue.
45	>	*
46	>	* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
47	>	* methods</i>, which are common in concurrent programming.  These methods,
48	>	* which wait for elements to appear or for space to become available, are
49	>	* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
50	>	* extends this interface.
51	>	*
52	>	* <p><tt>Queue</tt> implementations generally do not allow insertion
53	>	* of <tt>null</tt> elements, although some implementations, such as
54	>	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
55	>	* Even in the implementations that permit it, <tt>null</tt> should
56	>	* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
57	>	* used as a special return value by the <tt>poll</tt> method to
58	>	* indicate that the queue contains no elements.
59	>	*
60	>	* <p><tt>Queue</tt> implementations generally do not define
61	>	* element-based versions of methods <tt>equals</tt> and
62	>	* <tt>hashCode</tt> but instead inherit the identity based versions
63	>	* from class <tt>Object</tt>, because element-based equality is not
64	>	* always well-defined for queues with the same elements but different
65	>	* ordering properties.
66	>	*
67	>	*
68	>	* <p>This interface is a member of the
69	>	* <a href="{@docRoot}/../guide/collections/index.html">
70	>	* Java Collections Framework</a>.
71	>	*
72	>	* @see java.util.Collection
73	>	* @see LinkedList
74	>	* @see PriorityQueue
75	>	* @see java.util.concurrent.LinkedBlockingQueue
76	>	* @see java.util.concurrent.BlockingQueue
77	>	* @see java.util.concurrent.ArrayBlockingQueue
78	>	* @see java.util.concurrent.LinkedBlockingQueue
79	>	* @see java.util.concurrent.PriorityBlockingQueue
80	>	* @since 1.5
81	>	* @author Doug Lea
82	>	* @param <E> the type of elements held in this collection
83	>	*/
84		public interface Queue<E> extends Collection<E> {
85
86		/**
87	<	* Add the given object to this queue if possible.
88	<	* @param x the object to add
89	<	* @return true if successful
90	<	**/
91	<	public boolean offer(E x);
87	>	* Inserts the specified element into this queue, if possible.  When
88	>	* using queues that may impose insertion restrictions (for
89	>	* example capacity bounds), method <tt>offer</tt> is generally
90	>	* preferable to method {@link Collection#add}, which can fail to
91	>	* insert an element only by throwing an exception.
92	>	*
93	>	* @param o the element to insert.
94	>	* @return <tt>true</tt> if it was possible to add the element to
95	>	* this queue, else <tt>false</tt>
96	>	*/
97	>	boolean offer(E o);
98
99		/**
100	<	* Delete and return an object from the queue if one is available.
101	<	* @return the object, or null if the queue is empty.
102	<	**/
103	<	public E poll();
100	>	* Retrieves and removes the head of this queue, or <tt>null</tt>
101	>	* if this queue is empty.
102	>	*
103	>	* @return the head of this queue, or <tt>null</tt> if this
104	>	*         queue is empty.
105	>	*/
106	>	E poll();
107
108		/**
109	<	* Delete and return the element produced by poll, if the queue is
110	<	* not empty.
111	<	* @return an element
112	<	* @throws NoSuchElementException if empty
113	<	**/
114	<	public E remove() throws NoSuchElementException;
109	>	* Retrieves and removes the head of this queue.  This method
110	>	* differs from the <tt>poll</tt> method in that it throws an
111	>	* exception if this queue is empty.
112	>	*
113	>	* @return the head of this queue.
114	>	* @throws NoSuchElementException if this queue is empty.
115	>	*/
116	>	E remove();
117
118		/**
119	<	* Return but do not delete the element that will be returned by
120	<	* the next call to poll.
121	<	* @return an element, or null if empty
122	<	**/
123	<	public E peek();
119	>	* Retrieves, but does not remove, the head of this queue,
120	>	* returning <tt>null</tt> if this queue is empty.
121	>	*
122	>	* @return the head of this queue, or <tt>null</tt> if this queue
123	>	* is empty.
124	>	*/
125	>	E peek();
126
127		/**
128	<	* Return but do not delete the element that will be returned by
129	<	* the next call to poll, if the queue is not empty.
130	<	* @return an element
131	<	* @throws NoSuchElementException if empty
132	<	**/
133	<	public E element() throws NoSuchElementException;
128	>	* Retrieves, but does not remove, the head of this queue.  This method
129	>	* differs from the <tt>peek</tt> method only in that it throws an
130	>	* exception if this queue is empty.
131	>	*
132	>	* @return the head of this queue.
133	>	* @throws NoSuchElementException if this queue is empty.
134	>	*/
135	>	E element();
136		}

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