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Comparing jsr166/src/main/java/util/Queue.java (file contents):
Revision 1.1 by tim, Wed May 14 21:30:45 2003 UTC vs.
Revision 1.49 by jsr166, Wed Aug 23 20:17:53 2017 UTC

#	Line 1 | Line 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	<	* Queues are Collections supporting additional basic insertion,
11	<	* extraction, and inspection operations.
12	<	*
13	<	* <p> Queues typically, but do not necessarily order elements in a
14	<	* FIFO (first-in-first-out) manner. Among the exceptions are priority
15	<	* queues, that order elements in accord with supplied
16	<	* Comparators. Every Queue implementation must specify its ordering
17	<	* guarantees,
18	<	*
19	<	* <p> The <tt>offer</tt> method adds an element if possible,
20	<	* otherwise returning <tt>false</tt>. This differs from the
21	<	* Collections.add method, that throws an unchecked exception upon
22	<	* failure. It is designed for use in collections in which failure to
23	<	* add is a normal, rather than exceptional occurrence, for example,
24	<	* in fixed-capacity queues.
25	<	*
26	<	* <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return
27	<	* an element in accord with the implementation's ordering policies --
28	<	* for example, in FIFO queues, it will return the oldest element.
29	<	* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior
30	<	* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while
31	<	* <tt>remove</tt> throws an exception. These are designed for usage
32	<	* contexts in which emptiness is considered to be normal versus
33	<	* exceptional.
34	<	*
35	<	* <p> The <tt>element</tt> and <tt>peek</tt> methods return but do
36	<	* not delete the element that would be obtained by a call to
37	<	* <tt>remove</tt> and <tt>poll</tt> respectively.
38	<	*
39	<	* <p> The Queue interface does not define blocking queue methods
40	<	* (i.e., those that wait for elements to appear and/or for space to
41	<	* be available) that are common in concurrent programming. These are
42	<	* defined in the extended java.util.concurrent.BlockingQueue
43	<	* interface.
44	<	*
45	<	* <p> Queue implementations generally do not allow insertion of
46	<	* <tt>null</tt>. Even in those that allow it, it is a very bad idea
47	<	* to do so, since <tt>null</tt> is also used as a sentinel by
48	<	* <tt>poll</tt> to indicate that no elements exist.
49	<	**/
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 class="striped">
21	>	* <caption>Summary of Queue methods</caption>
22	>	*  <thead>
23	>	*  <tr>
24	>	*    <td></td>
25	>	*    <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
26	>	*    <th scope="col" style="font-weight:normal; font-style:italic">Returns special value</th>
27	>	*  </tr>
28	>	*  </thead>
29	>	*  <tbody>
30	>	*  <tr>
31	>	*    <th scope="row">Insert</th>
32	>	*    <td>{@link #add(Object) add(e)}</td>
33	>	*    <td>{@link #offer(Object) offer(e)}</td>
34	>	*  </tr>
35	>	*  <tr>
36	>	*    <th scope="row">Remove</th>
37	>	*    <td>{@link #remove() remove()}</td>
38	>	*    <td>{@link #poll() poll()}</td>
39	>	*  </tr>
40	>	*  <tr>
41	>	*    <th scope="row">Examine</th>
42	>	*    <td>{@link #element() element()}</td>
43	>	*    <td>{@link #peek() peek()}</td>
44	>	*  </tr>
45	>	*  </tbody>
46	>	* </table>
47	>	*
48	>	* <p>Queues typically, but do not necessarily, order elements in a
49	>	* FIFO (first-in-first-out) manner.  Among the exceptions are
50	>	* priority queues, which order elements according to a supplied
51	>	* comparator, or the elements' natural ordering, and LIFO queues (or
52	>	* stacks) which order the elements LIFO (last-in-first-out).
53	>	* Whatever the ordering used, the <em>head</em> of the queue is that
54	>	* element which would be removed by a call to {@link #remove()} or
55	>	* {@link #poll()}.  In a FIFO queue, all new elements are inserted at
56	>	* the <em>tail</em> of the queue. Other kinds of queues may use
57	>	* different placement rules.  Every {@code Queue} implementation
58	>	* must specify its ordering properties.
59	>	*
60	>	* <p>The {@link #offer offer} method inserts an element if possible,
61	>	* otherwise returning {@code false}.  This differs from the {@link
62	>	* java.util.Collection#add Collection.add} method, which can fail to
63	>	* add an element only by throwing an unchecked exception.  The
64	>	* {@code offer} method is designed for use when failure is a normal,
65	>	* rather than exceptional occurrence, for example, in fixed-capacity
66	>	* (or &quot;bounded&quot;) queues.
67	>	*
68	>	* <p>The {@link #remove()} and {@link #poll()} methods remove and
69	>	* return the head of the queue.
70	>	* Exactly which element is removed from the queue is a
71	>	* function of the queue's ordering policy, which differs from
72	>	* implementation to implementation. The {@code remove()} and
73	>	* {@code poll()} methods differ only in their behavior when the
74	>	* queue is empty: the {@code remove()} method throws an exception,
75	>	* while the {@code poll()} method returns {@code null}.
76	>	*
77	>	* <p>The {@link #element()} and {@link #peek()} methods return, but do
78	>	* not remove, the head of the queue.
79	>	*
80	>	* <p>The {@code Queue} interface does not define the <i>blocking queue
81	>	* methods</i>, which are common in concurrent programming.  These methods,
82	>	* which wait for elements to appear or for space to become available, are
83	>	* defined in the {@link java.util.concurrent.BlockingQueue} interface, which
84	>	* extends this interface.
85	>	*
86	>	* <p>{@code Queue} implementations generally do not allow insertion
87	>	* of {@code null} elements, although some implementations, such as
88	>	* {@link LinkedList}, do not prohibit insertion of {@code null}.
89	>	* Even in the implementations that permit it, {@code null} should
90	>	* not be inserted into a {@code Queue}, as {@code null} is also
91	>	* used as a special return value by the {@code poll} method to
92	>	* indicate that the queue contains no elements.
93	>	*
94	>	* <p>{@code Queue} implementations generally do not define
95	>	* element-based versions of methods {@code equals} and
96	>	* {@code hashCode} but instead inherit the identity based versions
97	>	* from class {@code Object}, because element-based equality is not
98	>	* always well-defined for queues with the same elements but different
99	>	* ordering properties.
100	>	*
101	>	* <p>This interface is a member of the
102	>	* <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
103	>	* Java Collections Framework</a>.
104	>	*
105	>	* @since 1.5
106	>	* @author Doug Lea
107	>	* @param <E> the type of elements held in this queue
108	>	*/
109		public interface Queue<E> extends Collection<E> {
110	+	/**
111	+	* Inserts the specified element into this queue if it is possible to do so
112	+	* immediately without violating capacity restrictions, returning
113	+	* {@code true} upon success and throwing an {@code IllegalStateException}
114	+	* if no space is currently available.
115	+	*
116	+	* @param e the element to add
117	+	* @return {@code true} (as specified by {@link Collection#add})
118	+	* @throws IllegalStateException if the element cannot be added at this
119	+	*         time due to capacity restrictions
120	+	* @throws ClassCastException if the class of the specified element
121	+	*         prevents it from being added to this queue
122	+	* @throws NullPointerException if the specified element is null and
123	+	*         this queue does not permit null elements
124	+	* @throws IllegalArgumentException if some property of this element
125	+	*         prevents it from being added to this queue
126	+	*/
127	+	boolean add(E e);
128	+
129	+	/**
130	+	* Inserts the specified element into this queue if it is possible to do
131	+	* so immediately without violating capacity restrictions.
132	+	* When using a capacity-restricted queue, this method is generally
133	+	* preferable to {@link #add}, which can fail to insert an element only
134	+	* by throwing an exception.
135	+	*
136	+	* @param e the element to add
137	+	* @return {@code true} if the element was added to this queue, else
138	+	*         {@code false}
139	+	* @throws ClassCastException if the class of the specified element
140	+	*         prevents it from being added to this queue
141	+	* @throws NullPointerException if the specified element is null and
142	+	*         this queue does not permit null elements
143	+	* @throws IllegalArgumentException if some property of this element
144	+	*         prevents it from being added to this queue
145	+	*/
146	+	boolean offer(E e);
147	+
148	+	/**
149	+	* Retrieves and removes the head of this queue.  This method differs
150	+	* from {@link #poll() poll()} only in that it throws an exception if
151	+	* this queue is empty.
152	+	*
153	+	* @return the head of this queue
154	+	* @throws NoSuchElementException if this queue is empty
155	+	*/
156	+	E remove();
157	+
158	+	/**
159	+	* Retrieves and removes the head of this queue,
160	+	* or returns {@code null} if this queue is empty.
161	+	*
162	+	* @return the head of this queue, or {@code null} if this queue is empty
163	+	*/
164	+	E poll();
165	+
166	+	/**
167	+	* Retrieves, but does not remove, the head of this queue.  This method
168	+	* differs from {@link #peek peek} only in that it throws an exception
169	+	* if this queue is empty.
170	+	*
171	+	* @return the head of this queue
172	+	* @throws NoSuchElementException if this queue is empty
173	+	*/
174	+	E element();
175
176		/**
177	<	* Add the given object to this queue if possible.
178	<	* @param x the object to add
179	<	* @return true if successful
180	<	**/
181	<	public boolean offer(E x);
182	<
53	<	/**
54	<	* Delete and return an object from the queue if one is available.
55	<	* @return the object, or null if the queue is empty.
56	<	**/
57	<	public E poll();
58	<
59	<	/**
60	<	* Delete and return the element produced by poll, if the queue is
61	<	* not empty.
62	<	* @return an element
63	<	* @throws NoSuchElementException if empty
64	<	**/
65	<	public E remove() throws NoSuchElementException;
66	<
67	<	/**
68	<	* Return but do not delete the element that will be returned by
69	<	* the next call to poll.
70	<	* @return an element, or null if empty
71	<	**/
72	<	public E peek();
73	<
74	<	/**
75	<	* Return but do not delete the element that will be returned by
76	<	* the next call to poll, if the queue is not empty.
77	<	* @return an element
78	<	* @throws NoSuchElementException if empty
79	<	**/
80	<	public E element() throws NoSuchElementException;
177	>	* Retrieves, but does not remove, the head of this queue,
178	>	* or returns {@code null} if this queue is empty.
179	>	*
180	>	* @return the head of this queue, or {@code null} if this queue is empty
181	>	*/
182	>	E peek();
183		}

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