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 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 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:	1.19
Committed:	Mon Sep 15 12:02:23 2003 UTC (20 years, 7 months ago) by dl
Branch:	MAIN
Changes since 1.18:	+1 -1 lines
Log Message:	Fix some javadoc inconsistencies
#	Content
1	/*
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	package java.util;
8
9	/**
10	* A collection designed for holding elements prior to processing.
11	* Besides basic {@link java.util.Collection Collection} operations, queues provide
12	* additional insertion, extraction, and inspection operations.
13	*
14	* <p>Queues typically, but do not necessarily, order elements in a
15	* 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, 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
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	*
34	* <p>The {@link #remove()} and {@link #poll()} methods remove and
35	* return the head of the queue.
36	* Exactly which element is removed from the queue is a
37	* function of the queue's ordering policy, which differs from
38	* implementation to implementation. The <tt>remove()</tt> and
39	* <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	*
43	* <p>The {@link #element()} and {@link #peek()} methods return, but do
44	* not remove, the head of the queue.
45	*
46	* <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	* <p><tt>Queue</tt> implementations generally do not allow insertion
53	* of <tt>null</tt> elements, although some implementations, such as
54	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
55	* 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	*
60	* <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	* <p>This interface is a member of the
69	* <a href="{@docRoot}/../guide/collections/index.html">
70	* Java Collections Framework</a>.
71	*
72	* @see java.util.Collection
73	* @see LinkedList
74	* @see PriorityQueue
75	* @see java.util.concurrent.LinkedBlockingQueue
76	* @see java.util.concurrent.BlockingQueue
77	* @see java.util.concurrent.ArrayBlockingQueue
78	* @see java.util.concurrent.LinkedBlockingQueue
79	* @see java.util.concurrent.PriorityBlockingQueue
80	* @since 1.5
81	* @author Doug Lea
82	*/
83	public interface Queue<E> extends Collection<E> {
84
85	/**
86	* Inserts the specified element into 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	*
92	* @param o the element to insert.
93	* @return <tt>true</tt> if it was possible to add the element to
94	* this queue, else <tt>false</tt>
95	*/
96	boolean offer(E o);
97
98	/**
99	* Retrieves and removes the head of this queue, or <tt>null</tt>
100	* if this queue is empty.
101	*
102	* @return the head of this queue, or <tt>null</tt> if this
103	* queue is empty.
104	*/
105	E poll();
106
107	/**
108	* 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	*
112	* @return the head of this queue.
113	* @throws NoSuchElementException if this queue is empty.
114	*/
115	E remove();
116
117	/**
118	* Retrieves, but does not remove, the head of this queue,
119	* returning <tt>null</tt> if this queue is empty.
120	*
121	* @return the head of this queue, or <tt>null</tt> if this queue
122	* is empty.
123	*/
124	E peek();
125
126	/**
127	* Retrieves, but does not remove, the head of this queue. This method
128	* differs from the <tt>peek</tt> method only in that it throws an
129	* exception if this queue is empty.
130	*
131	* @return the head of this queue.
132	* @throws NoSuchElementException if this queue is empty.
133	*/
134	E element();
135	}