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 java.util.Collection 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
 * comparator, or the elements' natural ordering, and LIFO queues (or
 * stacks) which order the elements LIFO (last-in-first-out).
 * Whatever the ordering used, the <em>head</em> of the queue is that element
 * which would be removed by a call to {@link #remove() } or {@link #poll()}.
 * Every <tt>Queue</tt> implementation must specify its ordering guarantees.
 *
 * <p>The {@link #offer offer} method adds an element if possible, otherwise
 * returning <tt>false</tt>.  This differs from the 
 * {@link java.util.Collection#add Collection.add}
 * 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 &quot;bounded&quot;) queues.
 *
 * <p>The {@link #remove()} and {@link #poll()} methods remove and
 * return the head of the queue.
 * Exactly which element is removed from the queue is a
 * function of the queue's ordering policy, which differs from
 * implementation to implementation. 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 remove, the head of the queue.
 *
 * <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 java.util.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 o the element to add.
     * @return <tt>true</tt> if it was possible to add the element to
     * this queue, else <tt>false</tt>
     */
    boolean offer(E o);

    /**
     * Retrieve and remove the head of this queue, if it is available.
     *
     * @return the head of this queue, or <tt>null</tt> if this
     *         queue is empty.
     */
    E poll();

    /**
     * Retrieve and remove the head of this queue.
     * This method differs
     * from the <tt>poll</tt> method in that it throws an exception if this
     * queue is empty.
     *
     * @return the head of this queue.
     * @throws NoSuchElementException if this queue is empty.
     */
    E remove();

    /**
     * Retrieve, but do not remove, the head of this queue.
     * This method differs from the <tt>poll</tt>
     * method only in that this method does not remove the head element from
     * this queue.
     *
     * @return the head of this queue, or <tt>null</tt> if this queue is empty.
     */
    E peek();

    /**
     * Retrieve, but do not remove, the head of this queue.  This method
     * differs from the <tt>peek</tt> method only in that it throws an
     * exception if this queue is empty.
     *
     * @return the head of this queue.
     * @throws NoSuchElementException if this queue is empty.
     */
    E element();
}


Revision:	1.13
Committed:	Mon Aug 4 01:54:13 2003 UTC (20 years, 9 months ago) by dholmes
Branch:	MAIN
Changes since 1.12:	+3 -2 lines
Log Message:	Removed types from method cross-references - simplifies things when generics are concerned.
#	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	dholmes	1.8	* A collection designed for holding elements prior to processing.
11	tim	1.11	* Besides basic {@link java.util.Collection Collection} operations, queues provide
12	tim	1.2	* additional insertion, extraction, and inspection operations.
13	dholmes	1.8	*
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	dholmes	1.8	* 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	dholmes	1.10	* Every <tt>Queue</tt> implementation must specify its ordering guarantees.
22	tim	1.2	*
23	dholmes	1.13	* <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	tim	1.11	* method, which throws an unchecked exception upon
27	tim	1.1	* failure. It is designed for use in collections in which failure to
28			* add is a normal, rather than exceptional occurrence, for example,
29	dholmes	1.8	* in fixed-capacity (or "bounded") queues.
30	tim	1.9	*
31	dl	1.7	* <p>The {@link #remove()} and {@link #poll()} methods remove and
32	dholmes	1.8	* return the head of the queue.
33			* Exactly which element is removed from the queue is a
34	dl	1.7	* function of the queue's ordering policy, which differs from
35	dholmes	1.8	* implementation to implementation. The <tt>remove()</tt> and
36	dl	1.7	* <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	tim	1.1	*
40	dholmes	1.8	* <p>The {@link #element()} and {@link #peek()} methods return, but do
41	dholmes	1.10	* not remove, the head of the queue.
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	tim	1.11	* @see java.util.Collection
62	tim	1.2	* @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	dholmes	1.8
74	tim	1.1	/**
75	tim	1.2	* Add the specified element to this queue, if possible.
76			*
77	dholmes	1.10	* @param o the element to add.
78	tim	1.9	* @return <tt>true</tt> if it was possible to add the element to
79	dholmes	1.10	* this queue, else <tt>false</tt>
80	tim	1.2	*/
81	dholmes	1.10	boolean offer(E o);
82	tim	1.1
83			/**
84	dholmes	1.8	* Retrieve and remove the head of this queue, if it is available.
85	tim	1.2	*
86	dholmes	1.8	* @return the head of this queue, or <tt>null</tt> if this
87	tim	1.9	* queue is empty.
88	tim	1.2	*/
89	dl	1.7	E poll();
90	tim	1.1
91			/**
92	tim	1.9	* Retrieve and remove the head of this queue.
93	dholmes	1.8	* This method differs
94	dholmes	1.10	* from the <tt>poll</tt> method in that it throws an exception if this
95	tim	1.9	* queue is empty.
96	tim	1.2	*
97	dholmes	1.8	* @return the head of this queue.
98			* @throws NoSuchElementException if this queue is empty.
99	tim	1.2	*/
100	tim	1.9	E remove();
101	tim	1.1
102			/**
103	dholmes	1.10	* Retrieve, but do 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	dholmes	1.8	* this queue.
107	tim	1.2	*
108	dholmes	1.8	* @return the head of this queue, or <tt>null</tt> if this queue is empty.
109	tim	1.2	*/
110	dl	1.7	E peek();
111	tim	1.1
112			/**
113	dholmes	1.8	* Retrieve, but do not remove, the head of this queue. This method
114	tim	1.9	* differs from the <tt>peek</tt> method only in that it throws an
115	dholmes	1.8	* exception if this queue is empty.
116	tim	1.2	*
117	dholmes	1.8	* @return the head of this queue.
118			* @throws NoSuchElementException if this queue is empty.
119	tim	1.2	*/
120	tim	1.9	E element();
121	tim	1.1	}
122	dholmes	1.8
123
124
125
126
127
128
129
130
131