java/util/Queue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

package java.util;

/**
 * A Collection designed for holding elements prior to processing.
 * Besides basic {@link Collection} operations, queues provide
 * additional insertion, extraction, and inspection operations.
 *
 * <p>Queues typically, but do not necessarily, order elements in a
 * FIFO (first-in-first-out) manner.  Among the exceptions are priority
 * queues, which order elements according to a supplied comparators, or
 * the elements natural ordering.  Every Queue implementation must specify
 * its ordering guarantees.
 *
 * <p>The {@link #offer(E)} method adds an element if possible, otherwise
 * returning <tt>false</tt>.  This differs from the {@link
 * Collections#add(Object)} method, which throws an unchecked exception upon
 * failure. It is designed for use in collections in which failure to
 * add is a normal, rather than exceptional occurrence, for example,
 * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.
 * 
 * <p>The {@link #remove()} and {@link #poll()} methods remove and return an
 * element in accord with the implementation's ordering policy. For example,
 * in FIFO queues, they remove and return the oldest element in the queue.
 * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their
 * behavior when the queue is empty: the <tt>remove()</tt> method throws an
 * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
 *
 * <p>The {@link #element()} and {@link #peek()} methods return but do
 * not delete the element that would be obtained by a call to
 * the <tt>remove</tt> and <tt>poll</tt> methods respectively.
 *
 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
 * methods</i>, which are common in concurrent programming.  These methods,
 * which wait for elements to appear or for space to become available, are
 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
 * extends this interface.
 *
 * <p><tt>Queue</tt> implementations generally do not allow insertion of
 * <tt>null</tt> elements.  Even in the few implementations that permit it,
 * it is a bad idea, as <tt>null</tt> is also used as a special return value
 * by the <tt>poll</tt> method to indicate that the queue contains no
 * elements.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @see Collection
 * @see LinkedList
 * @see PriorityQueue
 * @see LinkedQueue
 * @see java.util.concurrent.BlockingQueue
 * @see java.util.concurrent.ArrayBlockingQueue
 * @see java.util.concurrent.LinkedBlockingQueue
 * @see java.util.concurrent.PriorityBlockingQueue
 */
public interface Queue<E> extends Collection<E> {
    /**
     * Add the specified element to this queue, if possible.
     *
     * @param element the element to add.
     * @return true if it was possible to add the element to the queue.
     */
    public boolean offer(E element);

    /**
     * Remove and return an element from the queue if one is available.
     * Exactly which element is removed from the queue is a function
     * of the queue's ordering policy, which differs from implementation
     * to implementation.  Possible orderings include (but are not limited
     * to) first-in-first-out (FIFO), element priority, and arbitrary.
     *
     * @return an element previously on the queue, or <tt>null</tt> if the
     *         queue is empty. 
     */
    public E poll();

    /**
     * Remove and return an element from the queue.  This method differs
     * from the <tt>poll</tt> method in that it throws an exception if the
     * queue is empty. 
     *
     * @return an element previously on the queue.
     * @throws NoSuchElementException if the queue is empty.
     */
    public E remove() throws NoSuchElementException;

    /**
     * Return, but do not remove, an element from the queue, or <tt>null</tt>
     * if the queue is empty.  This method returns the same object reference
     * that would be returned by by the <tt>poll</tt> method.  The two methods
     * differ in that this method does not remove the element from the queue.
     *
     * @return an element on the queue, or <tt>null</tt> if the queue is empty.
     */
    public E peek();

    /**
     * Return, but do not remove, an element from the queue.  This method
     * differs from the <tt>peek</tt> method in that it throws an exception if
     * the queue is empty.
     *
     * @return an element on the queue.
     * @throws NoSuchElementException if the queue is empty.
     */
    public E element() throws NoSuchElementException;
}
Revision:	1.3
Committed:	Tue May 27 18:20:06 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRERELEASE_0_1
Changes since 1.2:	+9 -3 lines
Log Message:	re-checkin initial implementations
#	User	Rev	Content
1	dl	1.3	/*
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	tim	1.1	package java.util;
8
9			/**
10	tim	1.2	* 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	tim	1.1	* failure. It is designed for use in collections in which failure to
24			* add is a normal, rather than exceptional occurrence, for example,
25	tim	1.2	* in fixed-capacity (or “bounded”) queues.
26	dl	1.3	*
27	tim	1.2	* <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	tim	1.1	*
34	tim	1.2	* <p>The {@link #element()} and {@link #peek()} methods return but do
35	tim	1.1	* not delete the element that would be obtained by a call to
36	tim	1.2	* the <tt>remove</tt> and <tt>poll</tt> methods respectively.
37	tim	1.1	*
38	tim	1.2	* <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	tim	1.1	public interface Queue<E> extends Collection<E> {
64			/**
65	tim	1.2	* 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	tim	1.1
72			/**
73	tim	1.2	* 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	dl	1.3	* queue is empty.
81	tim	1.2	*/
82	tim	1.1	public E poll();
83
84			/**
85	tim	1.2	* 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	dl	1.3	* queue is empty.
88	tim	1.2	*
89			* @return an element previously on the queue.
90			* @throws NoSuchElementException if the queue is empty.
91			*/
92	tim	1.1	public E remove() throws NoSuchElementException;
93
94			/**
95	tim	1.2	* 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	tim	1.1	public E peek();
103
104			/**
105	tim	1.2	* 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	tim	1.1	public E element() throws NoSuchElementException;
113			}