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()}.  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
 */
public interface Queue<E> extends Collection<E> {

    /**
     * Inserts the specified element to 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 o 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 o);

    /**
     * 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 <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();

    /**
     * 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 <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:	1.18
Committed:	Sat Sep 13 18:51:06 2003 UTC (20 years, 7 months ago) by dl
Branch:	MAIN
Changes since 1.17:	+9 -10 lines
Log Message:	Proofreading pass -- many minor adjustments
#	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	dl	1.17	* Whatever the ordering used, the <em>head</em> of the queue is that
20			* element which would be removed by a call to {@link #remove() } or
21			* {@link #poll()}. In a FIFO queue, all new elements are inserted at
22			* the <em> tail</em> of the queue. Other kinds of queues may use
23			* different placement rules. Every <tt>Queue</tt> implementation
24			* must specify its ordering properties.
25			*
26			* <p>The {@link #offer offer} method inserts an element if possible,
27			* otherwise returning <tt>false</tt>. This differs from the {@link
28			* java.util.Collection#add Collection.add} method, which can fail to
29			* add an element only by throwing an unchecked exception. The
30			* <tt>offer</tt> method is designed for use when failure is a normal,
31			* rather than exceptional occurrence, for example, in fixed-capacity
32			* (or "bounded") queues.
33	tim	1.9	*
34	dl	1.7	* <p>The {@link #remove()} and {@link #poll()} methods remove and
35	dholmes	1.8	* return the head of the queue.
36			* Exactly which element is removed from the queue is a
37	dl	1.7	* function of the queue's ordering policy, which differs from
38	dholmes	1.8	* implementation to implementation. The <tt>remove()</tt> and
39	dl	1.7	* <tt>poll()</tt> methods differ only in their behavior when the
40			* queue is empty: the <tt>remove()</tt> method throws an exception,
41			* while the <tt>poll()</tt> method returns <tt>null</tt>.
42	tim	1.1	*
43	dholmes	1.8	* <p>The {@link #element()} and {@link #peek()} methods return, but do
44	dholmes	1.10	* not remove, the head of the queue.
45	tim	1.1	*
46	tim	1.2	* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
47			* methods</i>, which are common in concurrent programming. These methods,
48			* which wait for elements to appear or for space to become available, are
49			* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
50			* extends this interface.
51			*
52	dl	1.7	* <p><tt>Queue</tt> implementations generally do not allow insertion
53			* of <tt>null</tt> elements, although some implementations, such as
54	brian	1.6	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
55	dl	1.7	* Even in the implementations that permit it, <tt>null</tt> should
56			* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
57			* used as a special return value by the <tt>poll</tt> method to
58			* indicate that the queue contains no elements.
59	tim	1.2	*
60	dl	1.16	* <p><tt>Queue</tt> implementations generally do not define
61			* element-based versions of methods <tt>equals</tt> and
62			* <tt>hashCode</tt> but instead inherit the identity based versions
63			* from class <tt>Object</tt>, because element-based equality is not
64			* always well-defined for queues with the same elements but different
65			* ordering properties.
66			*
67			*
68	tim	1.2	* <p>This interface is a member of the
69			* <a href="{@docRoot}/../guide/collections/index.html">
70			* Java Collections Framework</a>.
71			*
72	tim	1.11	* @see java.util.Collection
73	tim	1.2	* @see LinkedList
74			* @see PriorityQueue
75	dholmes	1.15	* @see java.util.concurrent.LinkedBlockingQueue
76	tim	1.2	* @see java.util.concurrent.BlockingQueue
77			* @see java.util.concurrent.ArrayBlockingQueue
78			* @see java.util.concurrent.LinkedBlockingQueue
79			* @see java.util.concurrent.PriorityBlockingQueue
80	dl	1.7	* @since 1.5
81			* @author Doug Lea
82	tim	1.2	*/
83	tim	1.1	public interface Queue<E> extends Collection<E> {
84	dholmes	1.8
85	tim	1.1	/**
86	dl	1.17	* Inserts the specified element to this queue, if possible. When
87			* using queues that may impose insertion restrictions (for
88			* example capacity bounds), method <tt>offer</tt> is generally
89			* preferable to method {@link Collection#add}, which can fail to
90			* insert an element only by throwing an exception.
91	tim	1.2	*
92	dl	1.17	* @param o the element to insert.
93	tim	1.9	* @return <tt>true</tt> if it was possible to add the element to
94	dholmes	1.10	* this queue, else <tt>false</tt>
95	tim	1.2	*/
96	dholmes	1.10	boolean offer(E o);
97	tim	1.1
98			/**
99	dl	1.18	* Retrieves and removes the head of this queue, or <tt>null</tt>
100			* if this queue is empty.
101	tim	1.2	*
102	dholmes	1.8	* @return the head of this queue, or <tt>null</tt> if this
103	tim	1.9	* queue is empty.
104	tim	1.2	*/
105	dl	1.7	E poll();
106	tim	1.1
107			/**
108	dl	1.18	* Retrieves and removes the head of this queue. This method
109			* differs from the <tt>poll</tt> method in that it throws an
110			* exception if this queue is empty.
111	tim	1.2	*
112	dholmes	1.8	* @return the head of this queue.
113			* @throws NoSuchElementException if this queue is empty.
114	tim	1.2	*/
115	tim	1.9	E remove();
116	tim	1.1
117			/**
118	dl	1.18	* Retrieves, but does not remove, the head of this queue,
119			* returning <tt>null</tt> if this queue is empty.
120	tim	1.2	*
121	dl	1.18	* @return the head of this queue, or <tt>null</tt> if this queue
122			* is empty.
123	tim	1.2	*/
124	dl	1.7	E peek();
125	tim	1.1
126			/**
127	dholmes	1.14	* Retrieves, but does not remove, the head of this queue. This method
128	tim	1.9	* differs from the <tt>peek</tt> method only in that it throws an
129	dholmes	1.8	* exception if this queue is empty.
130	tim	1.2	*
131	dholmes	1.8	* @return the head of this queue.
132			* @throws NoSuchElementException if this queue is empty.
133	tim	1.2	*/
134	tim	1.9	E element();
135	tim	1.1	}