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/publicdomain/zero/1.0/
 */

package java.util;

/**
 * A collection designed for holding elements prior to processing.
 * Besides basic {@link 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
 * {@code null} or {@code false}, depending on the operation).  The
 * latter form of the insert operation is designed specifically for
 * use with capacity-restricted {@code Queue} implementations; in most
 * implementations, insert operations cannot fail.
 *
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Summary of Queue methods</caption>
 *  <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(Object) add(e)}</td>
 *    <td>{@link #offer(Object) 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 {@code Queue} implementation
 * must specify its ordering properties.
 *
 * <p>The {@link #offer offer} method inserts an element if possible,
 * otherwise returning {@code false}.  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
 * {@code offer} 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 {@code remove()} and
 * {@code poll()} methods differ only in their behavior when the
 * queue is empty: the {@code remove()} method throws an exception,
 * while the {@code poll()} method returns {@code null}.
 *
 * <p>The {@link #element()} and {@link #peek()} methods return, but do
 * not remove, the head of the queue.
 *
 * <p>The {@code Queue} 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>{@code Queue} implementations generally do not allow insertion
 * of {@code null} elements, although some implementations, such as
 * {@link LinkedList}, do not prohibit insertion of {@code null}.
 * Even in the implementations that permit it, {@code null} should
 * not be inserted into a {@code Queue}, as {@code null} is also
 * used as a special return value by the {@code poll} method to
 * indicate that the queue contains no elements.
 *
 * <p>{@code Queue} implementations generally do not define
 * element-based versions of methods {@code equals} and
 * {@code hashCode} but instead inherit the identity based versions
 * from class {@code Object}, 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}/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this queue
 */
public interface Queue<E> extends Collection<E> {
    /**
     * Inserts the specified element into this queue if it is possible to do so
     * immediately without violating capacity restrictions, returning
     * {@code true} upon success and throwing an {@code IllegalStateException}
     * if no space is currently available.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to capacity restrictions
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this queue
     * @throws NullPointerException if the specified element is null and
     *         this queue does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this queue
     */
    boolean add(E e);

    /**
     * Inserts the specified element into this queue if it is possible to do
     * so immediately without violating capacity restrictions.
     * When using a capacity-restricted queue, this method is generally
     * preferable to {@link #add}, which can fail to insert an element only
     * by throwing an exception.
     *
     * @param e the element to add
     * @return {@code true} if the element was added to this queue, else
     *         {@code false}
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this queue
     * @throws NullPointerException if the specified element is null and
     *         this queue does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this queue
     */
    boolean offer(E e);

    /**
     * Retrieves and removes the head of this queue.  This method differs
     * from {@link #poll() poll()} 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 and removes the head of this queue,
     * or returns {@code null} if this queue is empty.
     *
     * @return the head of this queue, or {@code null} if this queue is empty
     */
    E poll();

    /**
     * Retrieves, but does not remove, the head of this queue.  This method
     * differs from {@link #peek peek} 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();

    /**
     * Retrieves, but does not remove, the head of this queue,
     * or returns {@code null} if this queue is empty.
     *
     * @return the head of this queue, or {@code null} if this queue is empty
     */
    E peek();
}
Revision:	1.46
Committed:	Sat May 6 06:49:46 2017 UTC (6 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.45:	+1 -1 lines
Log Message:	8177789: fix collections framework links to point to java.util package doc
#	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/publicdomain/zero/1.0/
5	*/
6
7	package java.util;
8
9	/**
10	* A collection designed for holding elements prior to processing.
11	* Besides basic {@link Collection} operations, queues provide
12	* additional insertion, extraction, and inspection operations.
13	* Each of these methods exists in two forms: one throws an exception
14	* if the operation fails, the other returns a special value (either
15	* {@code null} or {@code false}, depending on the operation). The
16	* latter form of the insert operation is designed specifically for
17	* use with capacity-restricted {@code Queue} implementations; in most
18	* implementations, insert operations cannot fail.
19	*
20	* <table BORDER CELLPADDING=3 CELLSPACING=1>
21	* <caption>Summary of Queue methods</caption>
22	* <tr>
23	* <td></td>
24	* <td ALIGN=CENTER><em>Throws exception</em></td>
25	* <td ALIGN=CENTER><em>Returns special value</em></td>
26	* </tr>
27	* <tr>
28	* <td><b>Insert</b></td>
29	* <td>{@link #add(Object) add(e)}</td>
30	* <td>{@link #offer(Object) offer(e)}</td>
31	* </tr>
32	* <tr>
33	* <td><b>Remove</b></td>
34	* <td>{@link #remove() remove()}</td>
35	* <td>{@link #poll() poll()}</td>
36	* </tr>
37	* <tr>
38	* <td><b>Examine</b></td>
39	* <td>{@link #element() element()}</td>
40	* <td>{@link #peek() peek()}</td>
41	* </tr>
42	* </table>
43	*
44	* <p>Queues typically, but do not necessarily, order elements in a
45	* FIFO (first-in-first-out) manner. Among the exceptions are
46	* priority queues, which order elements according to a supplied
47	* comparator, or the elements' natural ordering, and LIFO queues (or
48	* stacks) which order the elements LIFO (last-in-first-out).
49	* Whatever the ordering used, the <em>head</em> of the queue is that
50	* element which would be removed by a call to {@link #remove()} or
51	* {@link #poll()}. In a FIFO queue, all new elements are inserted at
52	* the <em>tail</em> of the queue. Other kinds of queues may use
53	* different placement rules. Every {@code Queue} implementation
54	* must specify its ordering properties.
55	*
56	* <p>The {@link #offer offer} method inserts an element if possible,
57	* otherwise returning {@code false}. This differs from the {@link
58	* java.util.Collection#add Collection.add} method, which can fail to
59	* add an element only by throwing an unchecked exception. The
60	* {@code offer} method is designed for use when failure is a normal,
61	* rather than exceptional occurrence, for example, in fixed-capacity
62	* (or "bounded") queues.
63	*
64	* <p>The {@link #remove()} and {@link #poll()} methods remove and
65	* return the head of the queue.
66	* Exactly which element is removed from the queue is a
67	* function of the queue's ordering policy, which differs from
68	* implementation to implementation. The {@code remove()} and
69	* {@code poll()} methods differ only in their behavior when the
70	* queue is empty: the {@code remove()} method throws an exception,
71	* while the {@code poll()} method returns {@code null}.
72	*
73	* <p>The {@link #element()} and {@link #peek()} methods return, but do
74	* not remove, the head of the queue.
75	*
76	* <p>The {@code Queue} interface does not define the <i>blocking queue
77	* methods</i>, which are common in concurrent programming. These methods,
78	* which wait for elements to appear or for space to become available, are
79	* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
80	* extends this interface.
81	*
82	* <p>{@code Queue} implementations generally do not allow insertion
83	* of {@code null} elements, although some implementations, such as
84	* {@link LinkedList}, do not prohibit insertion of {@code null}.
85	* Even in the implementations that permit it, {@code null} should
86	* not be inserted into a {@code Queue}, as {@code null} is also
87	* used as a special return value by the {@code poll} method to
88	* indicate that the queue contains no elements.
89	*
90	* <p>{@code Queue} implementations generally do not define
91	* element-based versions of methods {@code equals} and
92	* {@code hashCode} but instead inherit the identity based versions
93	* from class {@code Object}, because element-based equality is not
94	* always well-defined for queues with the same elements but different
95	* ordering properties.
96	*
97	* <p>This interface is a member of the
98	* <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
99	* Java Collections Framework</a>.
100	*
101	* @since 1.5
102	* @author Doug Lea
103	* @param <E> the type of elements held in this queue
104	*/
105	public interface Queue<E> extends Collection<E> {
106	/**
107	* Inserts the specified element into this queue if it is possible to do so
108	* immediately without violating capacity restrictions, returning
109	* {@code true} upon success and throwing an {@code IllegalStateException}
110	* if no space is currently available.
111	*
112	* @param e the element to add
113	* @return {@code true} (as specified by {@link Collection#add})
114	* @throws IllegalStateException if the element cannot be added at this
115	* time due to capacity restrictions
116	* @throws ClassCastException if the class of the specified element
117	* prevents it from being added to this queue
118	* @throws NullPointerException if the specified element is null and
119	* this queue does not permit null elements
120	* @throws IllegalArgumentException if some property of this element
121	* prevents it from being added to this queue
122	*/
123	boolean add(E e);
124
125	/**
126	* Inserts the specified element into this queue if it is possible to do
127	* so immediately without violating capacity restrictions.
128	* When using a capacity-restricted queue, this method is generally
129	* preferable to {@link #add}, which can fail to insert an element only
130	* by throwing an exception.
131	*
132	* @param e the element to add
133	* @return {@code true} if the element was added to this queue, else
134	* {@code false}
135	* @throws ClassCastException if the class of the specified element
136	* prevents it from being added to this queue
137	* @throws NullPointerException if the specified element is null and
138	* this queue does not permit null elements
139	* @throws IllegalArgumentException if some property of this element
140	* prevents it from being added to this queue
141	*/
142	boolean offer(E e);
143
144	/**
145	* Retrieves and removes the head of this queue. This method differs
146	* from {@link #poll() poll()} only in that it throws an exception if
147	* this queue is empty.
148	*
149	* @return the head of this queue
150	* @throws NoSuchElementException if this queue is empty
151	*/
152	E remove();
153
154	/**
155	* Retrieves and removes the head of this queue,
156	* or returns {@code null} if this queue is empty.
157	*
158	* @return the head of this queue, or {@code null} if this queue is empty
159	*/
160	E poll();
161
162	/**
163	* Retrieves, but does not remove, the head of this queue. This method
164	* differs from {@link #peek peek} only in that it throws an exception
165	* if this queue is empty.
166	*
167	* @return the head of this queue
168	* @throws NoSuchElementException if this queue is empty
169	*/
170	E element();
171
172	/**
173	* Retrieves, but does not remove, the head of this queue,
174	* or returns {@code null} if this queue is empty.
175	*
176	* @return the head of this queue, or {@code null} if this queue is empty
177	*/
178	E peek();
179	}