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
 */
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.
     *
     * @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:	1.6
Committed:	Mon Jun 23 02:26:15 2003 UTC (20 years, 10 months ago) by brian
Branch:	MAIN
Changes since 1.5:	+7 -5 lines
Log Message:	Partial javadoc pass
#	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	tim	1.2	* <p>The {@link #remove()} and {@link #poll()} methods remove and return an
29	dl	1.4	* element in accord with the implementation's ordering policy.
30			* Exactly which element is removed from the queue is a function
31			* of the queue's ordering policy, which differs from implementation
32			* to implementation. Possible orderings include (but are not limited
33	brian	1.6	* to) first-in-first-out (FIFO), last-in-first-out (LIFO), element priority, and arbitrary.
34	tim	1.2	* The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their
35			* behavior when the queue is empty: the <tt>remove()</tt> method throws an
36			* exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
37	tim	1.1	*
38	tim	1.2	* <p>The {@link #element()} and {@link #peek()} methods return but do
39	tim	1.1	* not delete the element that would be obtained by a call to
40	tim	1.2	* the <tt>remove</tt> and <tt>poll</tt> methods respectively.
41	tim	1.1	*
42	tim	1.2	* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
43			* methods</i>, which are common in concurrent programming. These methods,
44			* which wait for elements to appear or for space to become available, are
45			* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
46			* extends this interface.
47			*
48			* <p><tt>Queue</tt> implementations generally do not allow insertion of
49	brian	1.6	* <tt>null</tt> elements, although some implementations, such as
50			* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
51			* Even in the implementations that permit it, <tt>null</tt> should not be inserted into
52			* a <tt>Queue</tt>, as <tt>null</tt> is also used as a special return value
53	tim	1.2	* by the <tt>poll</tt> method to indicate that the queue contains no
54			* elements.
55			*
56			* <p>This interface is a member of the
57			* <a href="{@docRoot}/../guide/collections/index.html">
58			* Java Collections Framework</a>.
59			*
60			* @see Collection
61			* @see LinkedList
62			* @see PriorityQueue
63	brian	1.6	* @see java.util.concurrent.LinkedQueue
64	tim	1.2	* @see java.util.concurrent.BlockingQueue
65			* @see java.util.concurrent.ArrayBlockingQueue
66			* @see java.util.concurrent.LinkedBlockingQueue
67			* @see java.util.concurrent.PriorityBlockingQueue
68			*/
69	tim	1.1	public interface Queue<E> extends Collection<E> {
70			/**
71	tim	1.2	* Add the specified element to this queue, if possible.
72			*
73			* @param element the element to add.
74			* @return true if it was possible to add the element to the queue.
75			*/
76			public boolean offer(E element);
77	tim	1.1
78			/**
79	tim	1.2	* Remove and return an element from the queue if one is available.
80			*
81			* @return an element previously on the queue, or <tt>null</tt> if the
82	dl	1.3	* queue is empty.
83	tim	1.2	*/
84	tim	1.1	public E poll();
85
86			/**
87	tim	1.2	* Remove and return an element from the queue. This method differs
88			* from the <tt>poll</tt> method in that it throws an exception if the
89	dl	1.3	* queue is empty.
90	tim	1.2	*
91			* @return an element previously on the queue.
92			* @throws NoSuchElementException if the queue is empty.
93			*/
94	tim	1.1	public E remove() throws NoSuchElementException;
95
96			/**
97	tim	1.2	* Return, but do not remove, an element from the queue, or <tt>null</tt>
98			* if the queue is empty. This method returns the same object reference
99			* that would be returned by by the <tt>poll</tt> method. The two methods
100			* differ in that this method does not remove the element from the queue.
101			*
102			* @return an element on the queue, or <tt>null</tt> if the queue is empty.
103			*/
104	tim	1.1	public E peek();
105
106			/**
107	tim	1.2	* Return, but do not remove, an element from the queue. This method
108			* differs from the <tt>peek</tt> method in that it throws an exception if
109			* the queue is empty.
110			*
111			* @return an element on the queue.
112			* @throws NoSuchElementException if the queue is empty.
113			*/
114	tim	1.1	public E element() throws NoSuchElementException;
115			}