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 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 {@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.
 *
 * <p>
 * <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 Queue#add add(e)}</td>
 *    <td>{@link Queue#offer offer(e)}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Remove</b></td>
 *    <td>{@link Queue#remove remove()}</td>
 *    <td>{@link Queue#poll poll()}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Examine</b></td>
 *    <td>{@link Queue#element element()}</td>
 *    <td>{@link Queue#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}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @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 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:	1.5
Committed:	Fri Jul 15 18:49:12 2016 UTC (7 years, 9 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.4:	+0 -1 lines
Log Message:	whitespace
#	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 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 {@code null} or {@code false}, depending on the
16	* operation). The latter form of the insert operation is designed
17	* specifically for use with capacity-restricted {@code Queue}
18	* implementations; in most implementations, insert operations cannot
19	* fail.
20	*
21	* <p>
22	* <table BORDER CELLPADDING=3 CELLSPACING=1>
23	* <caption>Summary of Queue methods</caption>
24	* <tr>
25	* <td></td>
26	* <td ALIGN=CENTER><em>Throws exception</em></td>
27	* <td ALIGN=CENTER><em>Returns special value</em></td>
28	* </tr>
29	* <tr>
30	* <td><b>Insert</b></td>
31	* <td>{@link Queue#add add(e)}</td>
32	* <td>{@link Queue#offer offer(e)}</td>
33	* </tr>
34	* <tr>
35	* <td><b>Remove</b></td>
36	* <td>{@link Queue#remove remove()}</td>
37	* <td>{@link Queue#poll poll()}</td>
38	* </tr>
39	* <tr>
40	* <td><b>Examine</b></td>
41	* <td>{@link Queue#element element()}</td>
42	* <td>{@link Queue#peek peek()}</td>
43	* </tr>
44	* </table>
45	*
46	* <p>Queues typically, but do not necessarily, order elements in a
47	* FIFO (first-in-first-out) manner. Among the exceptions are
48	* priority queues, which order elements according to a supplied
49	* comparator, or the elements' natural ordering, and LIFO queues (or
50	* stacks) which order the elements LIFO (last-in-first-out).
51	* Whatever the ordering used, the <em>head</em> of the queue is that
52	* element which would be removed by a call to {@link #remove() } or
53	* {@link #poll()}. In a FIFO queue, all new elements are inserted at
54	* the <em>tail</em> of the queue. Other kinds of queues may use
55	* different placement rules. Every {@code Queue} implementation
56	* must specify its ordering properties.
57	*
58	* <p>The {@link #offer offer} method inserts an element if possible,
59	* otherwise returning {@code false}. This differs from the {@link
60	* java.util.Collection#add Collection.add} method, which can fail to
61	* add an element only by throwing an unchecked exception. The
62	* {@code offer} method is designed for use when failure is a normal,
63	* rather than exceptional occurrence, for example, in fixed-capacity
64	* (or "bounded") queues.
65	*
66	* <p>The {@link #remove()} and {@link #poll()} methods remove and
67	* return the head of the queue.
68	* Exactly which element is removed from the queue is a
69	* function of the queue's ordering policy, which differs from
70	* implementation to implementation. The {@code remove()} and
71	* {@code poll()} methods differ only in their behavior when the
72	* queue is empty: the {@code remove()} method throws an exception,
73	* while the {@code poll()} method returns {@code null}.
74	*
75	* <p>The {@link #element()} and {@link #peek()} methods return, but do
76	* not remove, the head of the queue.
77	*
78	* <p>The {@code Queue} interface does not define the <i>blocking queue
79	* methods</i>, which are common in concurrent programming. These methods,
80	* which wait for elements to appear or for space to become available, are
81	* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
82	* extends this interface.
83	*
84	* <p>{@code Queue} implementations generally do not allow insertion
85	* of {@code null} elements, although some implementations, such as
86	* {@link LinkedList}, do not prohibit insertion of {@code null}.
87	* Even in the implementations that permit it, {@code null} should
88	* not be inserted into a {@code Queue}, as {@code null} is also
89	* used as a special return value by the {@code poll} method to
90	* indicate that the queue contains no elements.
91	*
92	* <p>{@code Queue} implementations generally do not define
93	* element-based versions of methods {@code equals} and
94	* {@code hashCode} but instead inherit the identity based versions
95	* from class {@code Object}, because element-based equality is not
96	* always well-defined for queues with the same elements but different
97	* ordering properties.
98	*
99	* <p>This interface is a member of the
100	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
101	* Java Collections Framework</a>.
102	*
103	* @since 1.5
104	* @author Doug Lea
105	* @param <E> the type of elements held in this collection
106	*/
107	public interface Queue<E> extends Collection<E> {
108	/**
109	* Inserts the specified element into this queue if it is possible to do so
110	* immediately without violating capacity restrictions, returning
111	* {@code true} upon success and throwing an {@code IllegalStateException}
112	* if no space is currently available.
113	*
114	* @param e the element to add
115	* @return {@code true} (as specified by {@link Collection#add})
116	* @throws IllegalStateException if the element cannot be added at this
117	* time due to capacity restrictions
118	* @throws ClassCastException if the class of the specified element
119	* prevents it from being added to this queue
120	* @throws NullPointerException if the specified element is null and
121	* this queue does not permit null elements
122	* @throws IllegalArgumentException if some property of this element
123	* prevents it from being added to this queue
124	*/
125	boolean add(E e);
126
127	/**
128	* Inserts the specified element into this queue if it is possible to do
129	* so immediately without violating capacity restrictions.
130	* When using a capacity-restricted queue, this method is generally
131	* preferable to {@link #add}, which can fail to insert an element only
132	* by throwing an exception.
133	*
134	* @param e the element to add
135	* @return {@code true} if the element was added to this queue, else
136	* {@code false}
137	* @throws ClassCastException if the class of the specified element
138	* prevents it from being added to this queue
139	* @throws NullPointerException if the specified element is null and
140	* this queue does not permit null elements
141	* @throws IllegalArgumentException if some property of this element
142	* prevents it from being added to this queue
143	*/
144	boolean offer(E e);
145
146	/**
147	* Retrieves and removes the head of this queue. This method differs
148	* from {@link #poll poll} only in that it throws an exception if this
149	* queue is empty.
150	*
151	* @return the head of this queue
152	* @throws NoSuchElementException if this queue is empty
153	*/
154	E remove();
155
156	/**
157	* Retrieves and removes the head of this queue,
158	* or returns {@code null} if this queue is empty.
159	*
160	* @return the head of this queue, or {@code null} if this queue is empty
161	*/
162	E poll();
163
164	/**
165	* Retrieves, but does not remove, the head of this queue. This method
166	* differs from {@link #peek peek} only in that it throws an exception
167	* if this queue is empty.
168	*
169	* @return the head of this queue
170	* @throws NoSuchElementException if this queue is empty
171	*/
172	E element();
173
174	/**
175	* Retrieves, but does not remove, the head of this queue,
176	* or returns {@code null} if this queue is empty.
177	*
178	* @return the head of this queue, or {@code null} if this queue is empty
179	*/
180	E peek();
181	}