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.
 *
 * <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 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:	1.2
Committed:	Fri Jul 8 20:02:54 2016 UTC (7 years, 9 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+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	* <table BORDER CELLPADDING=3 CELLSPACING=1>
22	* <caption>Summary of Queue methods</caption>
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 Queue#add add(e)}</td>
31	* <td>{@link Queue#offer offer(e)}</td>
32	* </tr>
33	* <tr>
34	* <td><b>Remove</b></td>
35	* <td>{@link Queue#remove remove()}</td>
36	* <td>{@link Queue#poll poll()}</td>
37	* </tr>
38	* <tr>
39	* <td><b>Examine</b></td>
40	* <td>{@link Queue#element element()}</td>
41	* <td>{@link Queue#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 {@code Queue} implementation
55	* must specify its ordering properties.
56	*
57	* <p>The {@link #offer offer} method inserts an element if possible,
58	* otherwise returning {@code false}. 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	* {@code offer} 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 {@code remove()} and
70	* {@code poll()} methods differ only in their behavior when the
71	* queue is empty: the {@code remove()} method throws an exception,
72	* while the {@code poll()} method returns {@code null}.
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 {@code Queue} 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>{@code Queue} implementations generally do not allow insertion
84	* of {@code null} elements, although some implementations, such as
85	* {@link LinkedList}, do not prohibit insertion of {@code null}.
86	* Even in the implementations that permit it, {@code null} should
87	* not be inserted into a {@code Queue}, as {@code null} is also
88	* used as a special return value by the {@code poll} method to
89	* indicate that the queue contains no elements.
90	*
91	* <p>{@code Queue} implementations generally do not define
92	* element-based versions of methods {@code equals} and
93	* {@code hashCode} but instead inherit the identity based versions
94	* from class {@code Object}, because element-based equality is not
95	* always well-defined for queues with the same elements but different
96	* ordering properties.
97	*
98	* <p>This interface is a member of the
99	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
100	* Java Collections Framework</a>.
101	*
102	* @since 1.5
103	* @author Doug Lea
104	* @param <E> the type of elements held in this queue
105	*/
106	public interface Queue<E> extends Collection<E> {
107	/**
108	* Inserts the specified element into this queue if it is possible to do so
109	* immediately without violating capacity restrictions, returning
110	* {@code true} upon success and throwing an {@code IllegalStateException}
111	* if no space is currently available.
112	*
113	* @param e the element to add
114	* @return {@code true} (as specified by {@link Collection#add})
115	* @throws IllegalStateException if the element cannot be added at this
116	* time due to capacity restrictions
117	* @throws ClassCastException if the class of the specified element
118	* prevents it from being added to this queue
119	* @throws NullPointerException if the specified element is null and
120	* this queue does not permit null elements
121	* @throws IllegalArgumentException if some property of this element
122	* prevents it from being added to this queue
123	*/
124	boolean add(E e);
125
126	/**
127	* Inserts the specified element into this queue if it is possible to do
128	* so immediately without violating capacity restrictions.
129	* When using a capacity-restricted queue, this method is generally
130	* preferable to {@link #add}, which can fail to insert an element only
131	* by throwing an exception.
132	*
133	* @param e the element to add
134	* @return {@code true} if the element was added to this queue, else
135	* {@code false}
136	* @throws ClassCastException if the class of the specified element
137	* prevents it from being added to this queue
138	* @throws NullPointerException if the specified element is null and
139	* this queue does not permit null elements
140	* @throws IllegalArgumentException if some property of this element
141	* prevents it from being added to this queue
142	*/
143	boolean offer(E e);
144
145	/**
146	* Retrieves and removes the head of this queue. This method differs
147	* from {@link #poll poll} only in that it throws an exception if this
148	* queue is empty.
149	*
150	* @return the head of this queue
151	* @throws NoSuchElementException if this queue is empty
152	*/
153	E remove();
154
155	/**
156	* Retrieves and removes the head of this queue,
157	* or returns {@code null} if this queue is empty.
158	*
159	* @return the head of this queue, or {@code null} if this queue is empty
160	*/
161	E poll();
162
163	/**
164	* Retrieves, but does not remove, the head of this queue. This method
165	* differs from {@link #peek peek} only in that it throws an exception
166	* if this queue is empty.
167	*
168	* @return the head of this queue
169	* @throws NoSuchElementException if this queue is empty
170	*/
171	E element();
172
173	/**
174	* Retrieves, but does not remove, the head of this queue,
175	* or returns {@code null} if this queue is empty.
176	*
177	* @return the head of this queue, or {@code null} if this queue is empty
178	*/
179	E peek();
180	}