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><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> {

    /**
     * Adds 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);

    /**
     * Retrieves and removes 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();

    /**
     * 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.
     * 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();

    /**
     * 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:	1.16
Committed:	Sat Aug 30 11:40:04 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.15:	+8 -0 lines
Log Message:	Improve PQ.remove; mention that Queues don't usually define equals, hashCode
#	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	dl	1.16	* <p><tt>Queue</tt> implementations generally do not define
58			* element-based versions of methods <tt>equals</tt> and
59			* <tt>hashCode</tt> but instead inherit the identity based versions
60			* from class <tt>Object</tt>, because element-based equality is not
61			* always well-defined for queues with the same elements but different
62			* ordering properties.
63			*
64			*
65	tim	1.2	* <p>This interface is a member of the
66			* <a href="{@docRoot}/../guide/collections/index.html">
67			* Java Collections Framework</a>.
68			*
69	tim	1.11	* @see java.util.Collection
70	tim	1.2	* @see LinkedList
71			* @see PriorityQueue
72	dholmes	1.15	* @see java.util.concurrent.LinkedBlockingQueue
73	tim	1.2	* @see java.util.concurrent.BlockingQueue
74			* @see java.util.concurrent.ArrayBlockingQueue
75			* @see java.util.concurrent.LinkedBlockingQueue
76			* @see java.util.concurrent.PriorityBlockingQueue
77	dl	1.7	* @since 1.5
78			* @author Doug Lea
79	tim	1.2	*/
80	tim	1.1	public interface Queue<E> extends Collection<E> {
81	dholmes	1.8
82	tim	1.1	/**
83	dholmes	1.14	* Adds the specified element to this queue, if possible.
84	tim	1.2	*
85	dholmes	1.10	* @param o the element to add.
86	tim	1.9	* @return <tt>true</tt> if it was possible to add the element to
87	dholmes	1.10	* this queue, else <tt>false</tt>
88	tim	1.2	*/
89	dholmes	1.10	boolean offer(E o);
90	tim	1.1
91			/**
92	dholmes	1.14	* Retrieves and removes the head of this queue, if it is available.
93	tim	1.2	*
94	dholmes	1.8	* @return the head of this queue, or <tt>null</tt> if this
95	tim	1.9	* queue is empty.
96	tim	1.2	*/
97	dl	1.7	E poll();
98	tim	1.1
99			/**
100	dholmes	1.14	* Retrieves and removes the head of this queue.
101	dholmes	1.8	* This method differs
102	dholmes	1.10	* from the <tt>poll</tt> method in that it throws an exception if this
103	tim	1.9	* queue is empty.
104	tim	1.2	*
105	dholmes	1.8	* @return the head of this queue.
106			* @throws NoSuchElementException if this queue is empty.
107	tim	1.2	*/
108	tim	1.9	E remove();
109	tim	1.1
110			/**
111	dholmes	1.14	* Retrieves, but does not remove, the head of this queue.
112	dholmes	1.10	* This method differs from the <tt>poll</tt>
113			* method only in that this method does not remove the head element from
114	dholmes	1.8	* this queue.
115	tim	1.2	*
116	dholmes	1.8	* @return the head of this queue, or <tt>null</tt> if this queue is empty.
117	tim	1.2	*/
118	dl	1.7	E peek();
119	tim	1.1
120			/**
121	dholmes	1.14	* Retrieves, but does not remove, the head of this queue. This method
122	tim	1.9	* differs from the <tt>peek</tt> method only in that it throws an
123	dholmes	1.8	* exception if this queue is empty.
124	tim	1.2	*
125	dholmes	1.8	* @return the head of this queue.
126			* @throws NoSuchElementException if this queue is empty.
127	tim	1.2	*/
128	tim	1.9	E element();
129	tim	1.1	}
130	dholmes	1.8
131
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133
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139