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, 10 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+0 -1 lines
Log Message:	whitespace
#	User	Rev	Content
1	jsr166	1.1	/*
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			}