java/util/Queue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

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.  Each of these methods exists in two forms: one throws
 * an exception if the operation fails, the other returns a special
 * value (either <tt>null</tt> or <tt>false</tt>, depending on the
 * operation).  The latter form of the insert operation is designed
 * specifically for use with capacity-restricted <tt>Queue</tt>
 * implementations; in most implementations, insert operations cannot
 * fail.
 *
 * <p>
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 *  <tr>
 *    <td></td>
 *    <td ALIGN=CENTER><em>Throws exception</em></td>
 *    <td ALIGN=CENTER><em>Returns special value</em></td>
 *  </tr>
 *  <tr>
 *    <td><b>Insert</b></td>
 *    <td>{@link #add add(e)}</td>
 *    <td>{@link #offer offer(e)}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Remove</b></td>
 *    <td>{@link #remove remove()}</td>
 *    <td>{@link #poll poll()}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Examine</b></td>
 *    <td>{@link #element element()}</td>
 *    <td>{@link #peek peek()}</td>
 *  </tr>
 * </table>
 *
 * <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()}.  In a FIFO queue, all new elements are inserted at
 * the <em> tail</em> of the queue. Other kinds of queues may use
 * different placement rules.  Every <tt>Queue</tt> implementation
 * must specify its ordering properties.
 *
 * <p>The {@link #offer offer} method inserts an element if possible,
 * otherwise returning <tt>false</tt>.  This differs from the {@link
 * java.util.Collection#add Collection.add} method, which can fail to
 * add an element only by throwing an unchecked exception.  The
 * <tt>offer</tt> method is designed for use when failure 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><tt>Queue</tt> implementations generally do not define
 * element-based versions of methods <tt>equals</tt> and
 * <tt>hashCode</tt> but instead inherit the identity based versions
 * from class <tt>Object</tt>, because element-based equality is not
 * always well-defined for queues with the same elements but different
 * ordering properties.
 *
 *
 * <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.LinkedBlockingQueue
 * @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
 * @param <E> the type of elements held in this collection
 */
public interface Queue<E> extends Collection<E> {

    /**
     * Inserts the specified element into this queue, if possible.  When
     * using queues that may impose insertion restrictions (for
     * example capacity bounds), method <tt>offer</tt> is generally
     * preferable to method {@link Collection#add}, which can fail to
     * insert an element only by throwing an exception.
     *
     * @param e the element to insert.
     * @return <tt>true</tt> if it was possible to add the element to
     * this queue, else <tt>false</tt>
     */
    boolean offer(E e);

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

    /**
     * Retrieves and removes the head of this queue.  This method
     * differs from the {@link #poll} 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 remove();

    /**
     * Retrieves, but does not remove, the head of this queue,
     * returning <tt>null</tt> if this queue is empty.
     *
     * @return the head of this queue, or <tt>null</tt> if this queue
     * is empty.
     */
    E peek();

    /**
     * Retrieves, but does not remove, the head of this queue.  This method
     * differs from the {@link #peek} 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:	1.28
Committed:	Mon May 2 17:34:02 2005 UTC (19 years ago) by jsr166
Branch:	MAIN
Changes since 1.27:	+2 -1 lines
Log Message:	<p> before <table>
#	User	Rev	Content
1	dl	1.3	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.22	* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/licenses/publicdomain
5	dl	1.3	*/
6
7	tim	1.1	package java.util;
8
9			/**
10	dholmes	1.8	* A collection designed for holding elements prior to processing.
11	dl	1.24	* Besides basic {@link java.util.Collection Collection} operations,
12			* queues provide additional insertion, extraction, and inspection
13			* operations. Each of these methods exists in two forms: one throws
14			* an exception if the operation fails, the other returns a special
15			* value (either <tt>null</tt> or <tt>false</tt>, depending on the
16			* operation). The latter form of the insert operation is designed
17			* specifically for use with capacity-restricted <tt>Queue</tt>
18			* implementations; in most implementations, insert operations cannot
19			* fail.
20	dl	1.23	*
21	jsr166	1.28	* <p>
22			* <table BORDER CELLPADDING=3 CELLSPACING=1>
23	dl	1.23	* <tr>
24	dl	1.24	* <td></td>
25			* <td ALIGN=CENTER><em>Throws exception</em></td>
26			* <td ALIGN=CENTER><em>Returns special value</em></td>
27	dl	1.23	* </tr>
28			* <tr>
29	dl	1.24	* <td><b>Insert</b></td>
30			* <td>{@link #add add(e)}</td>
31			* <td>{@link #offer offer(e)}</td>
32	dl	1.23	* </tr>
33			* <tr>
34	dl	1.24	* <td><b>Remove</b></td>
35			* <td>{@link #remove remove()}</td>
36			* <td>{@link #poll poll()}</td>
37	dl	1.23	* </tr>
38			* <tr>
39	dl	1.24	* <td><b>Examine</b></td>
40			* <td>{@link #element element()}</td>
41			* <td>{@link #peek peek()}</td>
42	dl	1.23	* </tr>
43	dl	1.24	* </table>
44	dholmes	1.8	*
45	tim	1.2	* <p>Queues typically, but do not necessarily, order elements in a
46	dl	1.5	* FIFO (first-in-first-out) manner. Among the exceptions are
47			* priority queues, which order elements according to a supplied
48	dholmes	1.8	* comparator, or the elements' natural ordering, and LIFO queues (or
49			* stacks) which order the elements LIFO (last-in-first-out).
50	dl	1.17	* Whatever the ordering used, the <em>head</em> of the queue is that
51			* element which would be removed by a call to {@link #remove() } or
52			* {@link #poll()}. In a FIFO queue, all new elements are inserted at
53			* the <em> tail</em> of the queue. Other kinds of queues may use
54			* different placement rules. Every <tt>Queue</tt> implementation
55			* must specify its ordering properties.
56			*
57			* <p>The {@link #offer offer} method inserts an element if possible,
58			* otherwise returning <tt>false</tt>. This differs from the {@link
59			* java.util.Collection#add Collection.add} method, which can fail to
60			* add an element only by throwing an unchecked exception. The
61			* <tt>offer</tt> method is designed for use when failure is a normal,
62			* rather than exceptional occurrence, for example, in fixed-capacity
63			* (or "bounded") queues.
64	tim	1.9	*
65	dl	1.7	* <p>The {@link #remove()} and {@link #poll()} methods remove and
66	dholmes	1.8	* return the head of the queue.
67			* Exactly which element is removed from the queue is a
68	dl	1.7	* function of the queue's ordering policy, which differs from
69	dholmes	1.8	* implementation to implementation. The <tt>remove()</tt> and
70	dl	1.7	* <tt>poll()</tt> methods differ only in their behavior when the
71			* queue is empty: the <tt>remove()</tt> method throws an exception,
72			* while the <tt>poll()</tt> method returns <tt>null</tt>.
73	tim	1.1	*
74	dholmes	1.8	* <p>The {@link #element()} and {@link #peek()} methods return, but do
75	dholmes	1.10	* not remove, the head of the queue.
76	tim	1.1	*
77	tim	1.2	* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
78			* methods</i>, which are common in concurrent programming. These methods,
79			* which wait for elements to appear or for space to become available, are
80			* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
81			* extends this interface.
82			*
83	dl	1.7	* <p><tt>Queue</tt> implementations generally do not allow insertion
84			* of <tt>null</tt> elements, although some implementations, such as
85	brian	1.6	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
86	dl	1.7	* Even in the implementations that permit it, <tt>null</tt> should
87			* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
88			* used as a special return value by the <tt>poll</tt> method to
89			* indicate that the queue contains no elements.
90	tim	1.2	*
91	dl	1.16	* <p><tt>Queue</tt> implementations generally do not define
92			* element-based versions of methods <tt>equals</tt> and
93			* <tt>hashCode</tt> but instead inherit the identity based versions
94			* from class <tt>Object</tt>, because element-based equality is not
95			* always well-defined for queues with the same elements but different
96			* ordering properties.
97			*
98			*
99	tim	1.2	* <p>This interface is a member of the
100			* <a href="{@docRoot}/../guide/collections/index.html">
101			* Java Collections Framework</a>.
102			*
103	tim	1.11	* @see java.util.Collection
104	tim	1.2	* @see LinkedList
105			* @see PriorityQueue
106	dholmes	1.15	* @see java.util.concurrent.LinkedBlockingQueue
107	tim	1.2	* @see java.util.concurrent.BlockingQueue
108			* @see java.util.concurrent.ArrayBlockingQueue
109			* @see java.util.concurrent.LinkedBlockingQueue
110			* @see java.util.concurrent.PriorityBlockingQueue
111	dl	1.7	* @since 1.5
112			* @author Doug Lea
113	dl	1.21	* @param <E> the type of elements held in this collection
114	tim	1.2	*/
115	tim	1.1	public interface Queue<E> extends Collection<E> {
116	dholmes	1.8
117	tim	1.1	/**
118	dl	1.19	* Inserts the specified element into this queue, if possible. When
119	dl	1.17	* using queues that may impose insertion restrictions (for
120			* example capacity bounds), method <tt>offer</tt> is generally
121			* preferable to method {@link Collection#add}, which can fail to
122			* insert an element only by throwing an exception.
123	tim	1.2	*
124	jsr166	1.27	* @param e the element to insert.
125	tim	1.9	* @return <tt>true</tt> if it was possible to add the element to
126	dholmes	1.10	* this queue, else <tt>false</tt>
127	tim	1.2	*/
128	jsr166	1.27	boolean offer(E e);
129	tim	1.1
130			/**
131	dl	1.18	* Retrieves and removes the head of this queue, or <tt>null</tt>
132			* if this queue is empty.
133	tim	1.2	*
134	dholmes	1.8	* @return the head of this queue, or <tt>null</tt> if this
135	tim	1.9	* queue is empty.
136	tim	1.2	*/
137	dl	1.7	E poll();
138	tim	1.1
139			/**
140	dl	1.18	* Retrieves and removes the head of this queue. This method
141	jsr166	1.26	* differs from the {@link #poll} method only in that it throws an
142	dl	1.18	* exception if this queue is empty.
143	tim	1.2	*
144	dholmes	1.8	* @return the head of this queue.
145			* @throws NoSuchElementException if this queue is empty.
146	tim	1.2	*/
147	tim	1.9	E remove();
148	tim	1.1
149			/**
150	dl	1.18	* Retrieves, but does not remove, the head of this queue,
151			* returning <tt>null</tt> if this queue is empty.
152	tim	1.2	*
153	dl	1.18	* @return the head of this queue, or <tt>null</tt> if this queue
154			* is empty.
155	tim	1.2	*/
156	dl	1.7	E peek();
157	tim	1.1
158			/**
159	dholmes	1.14	* Retrieves, but does not remove, the head of this queue. This method
160	jsr166	1.26	* differs from the {@link #peek} method only in that it throws an
161	dholmes	1.8	* exception if this queue is empty.
162	tim	1.2	*
163	dholmes	1.8	* @return the head of this queue.
164			* @throws NoSuchElementException if this queue is empty.
165	tim	1.2	*/
166	tim	1.9	E element();
167	tim	1.1	}