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.  Each of these methods exists in two forms: one throws
 * an exception if the operation fails, the other returns a special
 * value (either <tt>null</tt> or <tt>false</tt>, depending on the
 * operation).  The latter form of the insert operation is designed
 * specifically for use with capacity-restricted <tt>Queue</tt>
 * implementations; in most implementations, insert operations cannot
 * fail.
 *
 * <p>
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 *  <tr>
 *    <td></td>
 *    <td ALIGN=CENTER><em>Throws exception</em></td>
 *    <td ALIGN=CENTER><em>Returns special value</em></td>
 *  </tr>
 *  <tr>
 *    <td><b>Insert</b></td>
 *    <td>{@link #add add(e)}</td>
 *    <td>{@link #offer offer(e)}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Remove</b></td>
 *    <td>{@link #remove remove()}</td>
 *    <td>{@link #poll poll()}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Examine</b></td>
 *    <td>{@link #element element()}</td>
 *    <td>{@link #peek peek()}</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 e 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 e);

    /**
     * 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 {@link #poll} 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 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 {@link #peek} 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:	1.28
Committed:	Mon May 2 17:34:02 2005 UTC (19 years ago) by jsr166
Branch:	MAIN
Changes since 1.27:	+2 -1 lines
Log Message:	<p> before <table>
#	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,
12	* queues provide additional insertion, extraction, and inspection
13	* operations. Each of these methods exists in two forms: one throws
14	* an exception if the operation fails, the other returns a special
15	* value (either <tt>null</tt> or <tt>false</tt>, depending on the
16	* operation). The latter form of the insert operation is designed
17	* specifically for use with capacity-restricted <tt>Queue</tt>
18	* implementations; in most implementations, insert operations cannot
19	* fail.
20	*
21	* <p>
22	* <table BORDER CELLPADDING=3 CELLSPACING=1>
23	* <tr>
24	* <td></td>
25	* <td ALIGN=CENTER><em>Throws exception</em></td>
26	* <td ALIGN=CENTER><em>Returns special value</em></td>
27	* </tr>
28	* <tr>
29	* <td><b>Insert</b></td>
30	* <td>{@link #add add(e)}</td>
31	* <td>{@link #offer offer(e)}</td>
32	* </tr>
33	* <tr>
34	* <td><b>Remove</b></td>
35	* <td>{@link #remove remove()}</td>
36	* <td>{@link #poll poll()}</td>
37	* </tr>
38	* <tr>
39	* <td><b>Examine</b></td>
40	* <td>{@link #element element()}</td>
41	* <td>{@link #peek peek()}</td>
42	* </tr>
43	* </table>
44	*
45	* <p>Queues typically, but do not necessarily, order elements in a
46	* FIFO (first-in-first-out) manner. Among the exceptions are
47	* priority queues, which order elements according to a supplied
48	* comparator, or the elements' natural ordering, and LIFO queues (or
49	* stacks) which order the elements LIFO (last-in-first-out).
50	* Whatever the ordering used, the <em>head</em> of the queue is that
51	* element which would be removed by a call to {@link #remove() } or
52	* {@link #poll()}. In a FIFO queue, all new elements are inserted at
53	* the <em> tail</em> of the queue. Other kinds of queues may use
54	* different placement rules. Every <tt>Queue</tt> implementation
55	* must specify its ordering properties.
56	*
57	* <p>The {@link #offer offer} method inserts an element if possible,
58	* otherwise returning <tt>false</tt>. This differs from the {@link
59	* java.util.Collection#add Collection.add} method, which can fail to
60	* add an element only by throwing an unchecked exception. The
61	* <tt>offer</tt> method is designed for use when failure is a normal,
62	* rather than exceptional occurrence, for example, in fixed-capacity
63	* (or "bounded") queues.
64	*
65	* <p>The {@link #remove()} and {@link #poll()} methods remove and
66	* return the head of the queue.
67	* Exactly which element is removed from the queue is a
68	* function of the queue's ordering policy, which differs from
69	* implementation to implementation. The <tt>remove()</tt> and
70	* <tt>poll()</tt> methods differ only in their behavior when the
71	* queue is empty: the <tt>remove()</tt> method throws an exception,
72	* while the <tt>poll()</tt> method returns <tt>null</tt>.
73	*
74	* <p>The {@link #element()} and {@link #peek()} methods return, but do
75	* not remove, the head of the queue.
76	*
77	* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
78	* methods</i>, which are common in concurrent programming. These methods,
79	* which wait for elements to appear or for space to become available, are
80	* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
81	* extends this interface.
82	*
83	* <p><tt>Queue</tt> implementations generally do not allow insertion
84	* of <tt>null</tt> elements, although some implementations, such as
85	* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
86	* Even in the implementations that permit it, <tt>null</tt> should
87	* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
88	* used as a special return value by the <tt>poll</tt> method to
89	* indicate that the queue contains no elements.
90	*
91	* <p><tt>Queue</tt> implementations generally do not define
92	* element-based versions of methods <tt>equals</tt> and
93	* <tt>hashCode</tt> but instead inherit the identity based versions
94	* from class <tt>Object</tt>, because element-based equality is not
95	* always well-defined for queues with the same elements but different
96	* ordering properties.
97	*
98	*
99	* <p>This interface is a member of the
100	* <a href="{@docRoot}/../guide/collections/index.html">
101	* Java Collections Framework</a>.
102	*
103	* @see java.util.Collection
104	* @see LinkedList
105	* @see PriorityQueue
106	* @see java.util.concurrent.LinkedBlockingQueue
107	* @see java.util.concurrent.BlockingQueue
108	* @see java.util.concurrent.ArrayBlockingQueue
109	* @see java.util.concurrent.LinkedBlockingQueue
110	* @see java.util.concurrent.PriorityBlockingQueue
111	* @since 1.5
112	* @author Doug Lea
113	* @param <E> the type of elements held in this collection
114	*/
115	public interface Queue<E> extends Collection<E> {
116
117	/**
118	* Inserts the specified element into this queue, if possible. When
119	* using queues that may impose insertion restrictions (for
120	* example capacity bounds), method <tt>offer</tt> is generally
121	* preferable to method {@link Collection#add}, which can fail to
122	* insert an element only by throwing an exception.
123	*
124	* @param e the element to insert.
125	* @return <tt>true</tt> if it was possible to add the element to
126	* this queue, else <tt>false</tt>
127	*/
128	boolean offer(E e);
129
130	/**
131	* Retrieves and removes the head of this queue, or <tt>null</tt>
132	* if this queue is empty.
133	*
134	* @return the head of this queue, or <tt>null</tt> if this
135	* queue is empty.
136	*/
137	E poll();
138
139	/**
140	* Retrieves and removes the head of this queue. This method
141	* differs from the {@link #poll} method only in that it throws an
142	* exception if this queue is empty.
143	*
144	* @return the head of this queue.
145	* @throws NoSuchElementException if this queue is empty.
146	*/
147	E remove();
148
149	/**
150	* Retrieves, but does not remove, the head of this queue,
151	* returning <tt>null</tt> if this queue is empty.
152	*
153	* @return the head of this queue, or <tt>null</tt> if this queue
154	* is empty.
155	*/
156	E peek();
157
158	/**
159	* Retrieves, but does not remove, the head of this queue. This method
160	* differs from the {@link #peek} method only in that it throws an
161	* exception if this queue is empty.
162	*
163	* @return the head of this queue.
164	* @throws NoSuchElementException if this queue is empty.
165	*/
166	E element();
167	}