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. The names
 * of the operations vary with their policies:
 *
 *<table BORDER CELLPADDING=3 CELLSPACING=1>
 *  <tr>
 *    <td><em>Operation</em></td>
 *    <td ALIGN=CENTER><em>Attempt</em></td>
 *    <td ALIGN=CENTER><em>Throw</em></td>
 *  </tr>
 *  <tr>
 *    <td><em>insert</em></td>
 *    <td>offer(x)</td>
 *    <td>add(x)</td>
 *  </tr>
 *  <tr>
 *    <td><em>extract</em></td>
 *    <td>poll()</td>
 *    <td>remove()</td>
 *  </tr>
 *  <tr>
 *    <td><em>inspect</em></td>
 *    <td>peek()</td>
 *    <td>element()</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 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:	1.23
Committed:	Sun Nov 21 01:40:39 2004 UTC (19 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.22:	+25 -1 lines
Log Message:	Changes for maintenance/RFE phase
#	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	tim	1.11	* Besides basic {@link java.util.Collection Collection} operations, queues provide
12	dl	1.23	* additional insertion, extraction, and inspection operations. The names
13			* of the operations vary with their policies:
14			*
15			*<table BORDER CELLPADDING=3 CELLSPACING=1>
16			* <tr>
17			* <td><em>Operation</em></td>
18			* <td ALIGN=CENTER><em>Attempt</em></td>
19			* <td ALIGN=CENTER><em>Throw</em></td>
20			* </tr>
21			* <tr>
22			* <td><em>insert</em></td>
23			* <td>offer(x)</td>
24			* <td>add(x)</td>
25			* </tr>
26			* <tr>
27			* <td><em>extract</em></td>
28			* <td>poll()</td>
29			* <td>remove()</td>
30			* </tr>
31			* <tr>
32			* <td><em>inspect</em></td>
33			* <td>peek()</td>
34			* <td>element()</td>
35			* </tr>
36			*</table>
37	dholmes	1.8	*
38	tim	1.2	* <p>Queues typically, but do not necessarily, order elements in a
39	dl	1.5	* FIFO (first-in-first-out) manner. Among the exceptions are
40			* priority queues, which order elements according to a supplied
41	dholmes	1.8	* comparator, or the elements' natural ordering, and LIFO queues (or
42			* stacks) which order the elements LIFO (last-in-first-out).
43	dl	1.17	* Whatever the ordering used, the <em>head</em> of the queue is that
44			* element which would be removed by a call to {@link #remove() } or
45			* {@link #poll()}. In a FIFO queue, all new elements are inserted at
46			* the <em> tail</em> of the queue. Other kinds of queues may use
47			* different placement rules. Every <tt>Queue</tt> implementation
48			* must specify its ordering properties.
49			*
50			* <p>The {@link #offer offer} method inserts an element if possible,
51			* otherwise returning <tt>false</tt>. This differs from the {@link
52			* java.util.Collection#add Collection.add} method, which can fail to
53			* add an element only by throwing an unchecked exception. The
54			* <tt>offer</tt> method is designed for use when failure is a normal,
55			* rather than exceptional occurrence, for example, in fixed-capacity
56			* (or "bounded") queues.
57	tim	1.9	*
58	dl	1.7	* <p>The {@link #remove()} and {@link #poll()} methods remove and
59	dholmes	1.8	* return the head of the queue.
60			* Exactly which element is removed from the queue is a
61	dl	1.7	* function of the queue's ordering policy, which differs from
62	dholmes	1.8	* implementation to implementation. The <tt>remove()</tt> and
63	dl	1.7	* <tt>poll()</tt> methods differ only in their behavior when the
64			* queue is empty: the <tt>remove()</tt> method throws an exception,
65			* while the <tt>poll()</tt> method returns <tt>null</tt>.
66	tim	1.1	*
67	dholmes	1.8	* <p>The {@link #element()} and {@link #peek()} methods return, but do
68	dholmes	1.10	* not remove, the head of the queue.
69	tim	1.1	*
70	tim	1.2	* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
71			* methods</i>, which are common in concurrent programming. These methods,
72			* which wait for elements to appear or for space to become available, are
73			* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
74			* extends this interface.
75			*
76	dl	1.7	* <p><tt>Queue</tt> implementations generally do not allow insertion
77			* of <tt>null</tt> elements, although some implementations, such as
78	brian	1.6	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
79	dl	1.7	* Even in the implementations that permit it, <tt>null</tt> should
80			* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
81			* used as a special return value by the <tt>poll</tt> method to
82			* indicate that the queue contains no elements.
83	tim	1.2	*
84	dl	1.16	* <p><tt>Queue</tt> implementations generally do not define
85			* element-based versions of methods <tt>equals</tt> and
86			* <tt>hashCode</tt> but instead inherit the identity based versions
87			* from class <tt>Object</tt>, because element-based equality is not
88			* always well-defined for queues with the same elements but different
89			* ordering properties.
90			*
91			*
92	tim	1.2	* <p>This interface is a member of the
93			* <a href="{@docRoot}/../guide/collections/index.html">
94			* Java Collections Framework</a>.
95			*
96	tim	1.11	* @see java.util.Collection
97	tim	1.2	* @see LinkedList
98			* @see PriorityQueue
99	dholmes	1.15	* @see java.util.concurrent.LinkedBlockingQueue
100	tim	1.2	* @see java.util.concurrent.BlockingQueue
101			* @see java.util.concurrent.ArrayBlockingQueue
102			* @see java.util.concurrent.LinkedBlockingQueue
103			* @see java.util.concurrent.PriorityBlockingQueue
104	dl	1.7	* @since 1.5
105			* @author Doug Lea
106	dl	1.21	* @param <E> the type of elements held in this collection
107	tim	1.2	*/
108	tim	1.1	public interface Queue<E> extends Collection<E> {
109	dholmes	1.8
110	tim	1.1	/**
111	dl	1.19	* Inserts the specified element into this queue, if possible. When
112	dl	1.17	* using queues that may impose insertion restrictions (for
113			* example capacity bounds), method <tt>offer</tt> is generally
114			* preferable to method {@link Collection#add}, which can fail to
115			* insert an element only by throwing an exception.
116	tim	1.2	*
117	dl	1.17	* @param o the element to insert.
118	tim	1.9	* @return <tt>true</tt> if it was possible to add the element to
119	dholmes	1.10	* this queue, else <tt>false</tt>
120	tim	1.2	*/
121	dholmes	1.10	boolean offer(E o);
122	tim	1.1
123			/**
124	dl	1.18	* Retrieves and removes the head of this queue, or <tt>null</tt>
125			* if this queue is empty.
126	tim	1.2	*
127	dholmes	1.8	* @return the head of this queue, or <tt>null</tt> if this
128	tim	1.9	* queue is empty.
129	tim	1.2	*/
130	dl	1.7	E poll();
131	tim	1.1
132			/**
133	dl	1.18	* Retrieves and removes the head of this queue. This method
134			* differs from the <tt>poll</tt> method in that it throws an
135			* exception if this queue is empty.
136	tim	1.2	*
137	dholmes	1.8	* @return the head of this queue.
138			* @throws NoSuchElementException if this queue is empty.
139	tim	1.2	*/
140	tim	1.9	E remove();
141	tim	1.1
142			/**
143	dl	1.18	* Retrieves, but does not remove, the head of this queue,
144			* returning <tt>null</tt> if this queue is empty.
145	tim	1.2	*
146	dl	1.18	* @return the head of this queue, or <tt>null</tt> if this queue
147			* is empty.
148	tim	1.2	*/
149	dl	1.7	E peek();
150	tim	1.1
151			/**
152	dholmes	1.14	* Retrieves, but does not remove, the head of this queue. This method
153	tim	1.9	* differs from the <tt>peek</tt> method only in that it throws an
154	dholmes	1.8	* exception if this queue is empty.
155	tim	1.2	*
156	dholmes	1.8	* @return the head of this queue.
157			* @throws NoSuchElementException if this queue is empty.
158	tim	1.2	*/
159	tim	1.9	E element();
160	tim	1.1	}