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
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
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. 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	*
38	* <p>Queues typically, but do not necessarily, order elements in a
39	* FIFO (first-in-first-out) manner. Among the exceptions are
40	* priority queues, which order elements according to a supplied
41	* comparator, or the elements' natural ordering, and LIFO queues (or
42	* stacks) which order the elements LIFO (last-in-first-out).
43	* 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	*
58	* <p>The {@link #remove()} and {@link #poll()} methods remove and
59	* return the head of the queue.
60	* Exactly which element is removed from the queue is a
61	* function of the queue's ordering policy, which differs from
62	* implementation to implementation. The <tt>remove()</tt> and
63	* <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	*
67	* <p>The {@link #element()} and {@link #peek()} methods return, but do
68	* not remove, the head of the queue.
69	*
70	* <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	* <p><tt>Queue</tt> implementations generally do not allow insertion
77	* of <tt>null</tt> elements, although some implementations, such as
78	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
79	* 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	*
84	* <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	* <p>This interface is a member of the
93	* <a href="{@docRoot}/../guide/collections/index.html">
94	* Java Collections Framework</a>.
95	*
96	* @see java.util.Collection
97	* @see LinkedList
98	* @see PriorityQueue
99	* @see java.util.concurrent.LinkedBlockingQueue
100	* @see java.util.concurrent.BlockingQueue
101	* @see java.util.concurrent.ArrayBlockingQueue
102	* @see java.util.concurrent.LinkedBlockingQueue
103	* @see java.util.concurrent.PriorityBlockingQueue
104	* @since 1.5
105	* @author Doug Lea
106	* @param <E> the type of elements held in this collection
107	*/
108	public interface Queue<E> extends Collection<E> {
109
110	/**
111	* Inserts the specified element into this queue, if possible. When
112	* 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	*
117	* @param o the element to insert.
118	* @return <tt>true</tt> if it was possible to add the element to
119	* this queue, else <tt>false</tt>
120	*/
121	boolean offer(E o);
122
123	/**
124	* Retrieves and removes the head of this queue, or <tt>null</tt>
125	* if this queue is empty.
126	*
127	* @return the head of this queue, or <tt>null</tt> if this
128	* queue is empty.
129	*/
130	E poll();
131
132	/**
133	* 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	*
137	* @return the head of this queue.
138	* @throws NoSuchElementException if this queue is empty.
139	*/
140	E remove();
141
142	/**
143	* Retrieves, but does not remove, the head of this queue,
144	* returning <tt>null</tt> if this queue is empty.
145	*
146	* @return the head of this queue, or <tt>null</tt> if this queue
147	* is empty.
148	*/
149	E peek();
150
151	/**
152	* Retrieves, but does not remove, the head of this queue. This method
153	* differs from the <tt>peek</tt> method only in that it throws an
154	* exception if this queue is empty.
155	*
156	* @return the head of this queue.
157	* @throws NoSuchElementException if this queue is empty.
158	*/
159	E element();
160	}