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.
0 *
 * <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
 * comparator, 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.  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), last-in-first-out
 * (LIFO), element priority, and arbitrary.  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, although some implementations, such as
 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
 * Even in the implementations that permit it, <tt>null</tt> should
 * not be inserted into a <tt>Queue</tt>, 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 java.util.concurrent.LinkedQueue
 * @see java.util.concurrent.BlockingQueue
 * @see java.util.concurrent.ArrayBlockingQueue
 * @see java.util.concurrent.LinkedBlockingQueue
 * @see java.util.concurrent.PriorityBlockingQueue
 * @since 1.5
 * @author Doug Lea
 */
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.
     */
    boolean offer(E element);

    /**
     * Remove and return an element from the queue if one is available.
     *
     * @return an element previously on the queue, or <tt>null</tt> if the
     *         queue is empty. 
     */
    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.
     */
    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.
     */
    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.
     */
    E element() throws NoSuchElementException;
}
Revision:	1.7
Committed:	Tue Jun 24 14:34:30 2003 UTC (20 years, 10 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_2
Changes since 1.6:	+23 -20 lines
Log Message:	Added missing javadoc tags; minor reformatting
#	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	brian	1.6	0 *
14	tim	1.2	* <p>Queues typically, but do not necessarily, order elements in a
15	dl	1.5	* 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	tim	1.2	*
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	1.4
27	dl	1.3	*
28	dl	1.7	* <p>The {@link #remove()} and {@link #poll()} methods remove and
29			* return an element in accord with the implementation's ordering
30			* policy. Exactly which element is removed from the queue is a
31			* function of the queue's ordering policy, which differs from
32			* implementation to implementation. Possible orderings include (but
33			* are not limited to) first-in-first-out (FIFO), last-in-first-out
34			* (LIFO), element priority, and arbitrary. The <tt>remove()</tt> and
35			* <tt>poll()</tt> methods differ only in their behavior when the
36			* queue is empty: the <tt>remove()</tt> method throws an exception,
37			* while the <tt>poll()</tt> method returns <tt>null</tt>.
38	tim	1.1	*
39	tim	1.2	* <p>The {@link #element()} and {@link #peek()} methods return but do
40	tim	1.1	* not delete the element that would be obtained by a call to
41	tim	1.2	* the <tt>remove</tt> and <tt>poll</tt> methods respectively.
42	tim	1.1	*
43	tim	1.2	* <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	dl	1.7	* <p><tt>Queue</tt> implementations generally do not allow insertion
50			* of <tt>null</tt> elements, although some implementations, such as
51	brian	1.6	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
52	dl	1.7	* 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	tim	1.2	*
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 Collection
62			* @see LinkedList
63			* @see PriorityQueue
64	brian	1.6	* @see java.util.concurrent.LinkedQueue
65	tim	1.2	* @see java.util.concurrent.BlockingQueue
66			* @see java.util.concurrent.ArrayBlockingQueue
67			* @see java.util.concurrent.LinkedBlockingQueue
68			* @see java.util.concurrent.PriorityBlockingQueue
69	dl	1.7	* @since 1.5
70			* @author Doug Lea
71	tim	1.2	*/
72	tim	1.1	public interface Queue<E> extends Collection<E> {
73			/**
74	tim	1.2	* Add the specified element to this queue, if possible.
75			*
76			* @param element the element to add.
77			* @return true if it was possible to add the element to the queue.
78			*/
79	dl	1.7	boolean offer(E element);
80	tim	1.1
81			/**
82	tim	1.2	* Remove and return an element from the queue if one is available.
83			*
84			* @return an element previously on the queue, or <tt>null</tt> if the
85	dl	1.3	* queue is empty.
86	tim	1.2	*/
87	dl	1.7	E poll();
88	tim	1.1
89			/**
90	tim	1.2	* Remove and return an element from the queue. This method differs
91			* from the <tt>poll</tt> method in that it throws an exception if the
92	dl	1.3	* queue is empty.
93	tim	1.2	*
94			* @return an element previously on the queue.
95			* @throws NoSuchElementException if the queue is empty.
96			*/
97	dl	1.7	E remove() throws NoSuchElementException;
98	tim	1.1
99			/**
100	tim	1.2	* Return, but do not remove, an element from the queue, or <tt>null</tt>
101			* if the queue is empty. This method returns the same object reference
102			* that would be returned by by the <tt>poll</tt> method. The two methods
103			* differ in that this method does not remove the element from the queue.
104			*
105			* @return an element on the queue, or <tt>null</tt> if the queue is empty.
106			*/
107	dl	1.7	E peek();
108	tim	1.1
109			/**
110	tim	1.2	* Return, but do not remove, an element from the queue. This method
111			* differs from the <tt>peek</tt> method in that it throws an exception if
112			* the queue is empty.
113			*
114			* @return an element on the queue.
115			* @throws NoSuchElementException if the queue is empty.
116			*/
117	dl	1.7	E element() throws NoSuchElementException;
118	tim	1.1	}