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1 : dl 1.3 /*
2 :     * Written by Doug Lea with assistance from members of JCP JSR-166
3 : dl 1.22 * Expert Group and released to the public domain, as explained at
4 : jsr166 1.36 * http://creativecommons.org/publicdomain/zero/1.0/
5 : dl 1.3 */
6 :    
7 : tim 1.1 package java.util;
8 :    
9 :     /**
10 : dholmes 1.8 * A collection designed for holding elements prior to processing.
11 : jsr166 1.45 * 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 : dl 1.23 *
20 : jsr166 1.28 * <table BORDER CELLPADDING=3 CELLSPACING=1>
21 : jsr166 1.39 * <caption>Summary of Queue methods</caption>
22 : dl 1.23 * <tr>
23 : dl 1.24 * <td></td>
24 :     * <td ALIGN=CENTER><em>Throws exception</em></td>
25 : jsr166 1.32 * <td ALIGN=CENTER><em>Returns special value</em></td>
26 : dl 1.23 * </tr>
27 :     * <tr>
28 : dl 1.24 * <td><b>Insert</b></td>
29 : jsr166 1.45 * <td>{@link #add(Object) add(e)}</td>
30 :     * <td>{@link #offer(Object) offer(e)}</td>
31 : dl 1.23 * </tr>
32 :     * <tr>
33 : dl 1.24 * <td><b>Remove</b></td>
34 : jsr166 1.45 * <td>{@link #remove() remove()}</td>
35 :     * <td>{@link #poll() poll()}</td>
36 : dl 1.23 * </tr>
37 :     * <tr>
38 : dl 1.24 * <td><b>Examine</b></td>
39 : jsr166 1.45 * <td>{@link #element() element()}</td>
40 :     * <td>{@link #peek() peek()}</td>
41 : dl 1.23 * </tr>
42 : dl 1.24 * </table>
43 : dholmes 1.8 *
44 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
45 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
46 :     * priority queues, which order elements according to a supplied
47 : dholmes 1.8 * comparator, or the elements' natural ordering, and LIFO queues (or
48 :     * stacks) which order the elements LIFO (last-in-first-out).
49 : dl 1.17 * Whatever the ordering used, the <em>head</em> of the queue is that
50 : jsr166 1.45 * element which would be removed by a call to {@link #remove()} or
51 : dl 1.17 * {@link #poll()}. In a FIFO queue, all new elements are inserted at
52 : jsr166 1.37 * the <em>tail</em> of the queue. Other kinds of queues may use
53 : jsr166 1.38 * different placement rules. Every {@code Queue} implementation
54 : dl 1.17 * must specify its ordering properties.
55 :     *
56 :     * <p>The {@link #offer offer} method inserts an element if possible,
57 : jsr166 1.38 * otherwise returning {@code false}. This differs from the {@link
58 : dl 1.17 * java.util.Collection#add Collection.add} method, which can fail to
59 :     * add an element only by throwing an unchecked exception. The
60 : jsr166 1.38 * {@code offer} method is designed for use when failure is a normal,
61 : dl 1.17 * rather than exceptional occurrence, for example, in fixed-capacity
62 :     * (or &quot;bounded&quot;) queues.
63 : tim 1.9 *
64 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
65 : dholmes 1.8 * return the head of the queue.
66 :     * Exactly which element is removed from the queue is a
67 : dl 1.7 * function of the queue's ordering policy, which differs from
68 : jsr166 1.38 * 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 : tim 1.1 *
73 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
74 : dholmes 1.10 * not remove, the head of the queue.
75 : tim 1.1 *
76 : jsr166 1.38 * <p>The {@code Queue} interface does not define the <i>blocking queue
77 : tim 1.2 * 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 : jsr166 1.38 * <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 : dl 1.7 * indicate that the queue contains no elements.
89 : tim 1.2 *
90 : jsr166 1.38 * <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 : dl 1.16 * always well-defined for queues with the same elements but different
95 :     * ordering properties.
96 :     *
97 : tim 1.2 * <p>This interface is a member of the
98 : jsr166 1.35 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
99 : tim 1.2 * Java Collections Framework</a>.
100 :     *
101 : dl 1.7 * @since 1.5
102 :     * @author Doug Lea
103 : jsr166 1.42 * @param <E> the type of elements held in this queue
104 : tim 1.2 */
105 : tim 1.1 public interface Queue<E> extends Collection<E> {
106 :     /**
107 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do so
108 :     * immediately without violating capacity restrictions, returning
109 : jsr166 1.38 * {@code true} upon success and throwing an {@code IllegalStateException}
110 : jsr166 1.29 * if no space is currently available.
111 :     *
112 :     * @param e the element to add
113 : jsr166 1.38 * @return {@code true} (as specified by {@link Collection#add})
114 : jsr166 1.29 * @throws IllegalStateException if the element cannot be added at this
115 :     * time due to capacity restrictions
116 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
117 :     * prevents it from being added to this queue
118 : jsr166 1.29 * @throws NullPointerException if the specified element is null and
119 : jsr166 1.34 * this queue does not permit null elements
120 : jsr166 1.29 * @throws IllegalArgumentException if some property of this element
121 :     * prevents it from being added to this queue
122 : tim 1.2 */
123 : jsr166 1.29 boolean add(E e);
124 : tim 1.1
125 :     /**
126 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do
127 :     * so immediately without violating capacity restrictions.
128 : jsr166 1.31 * When using a capacity-restricted queue, this method is generally
129 : jsr166 1.29 * preferable to {@link #add}, which can fail to insert an element only
130 :     * by throwing an exception.
131 : tim 1.2 *
132 : jsr166 1.29 * @param e the element to add
133 : jsr166 1.38 * @return {@code true} if the element was added to this queue, else
134 :     * {@code false}
135 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
136 :     * prevents it from being added to this queue
137 : jsr166 1.29 * @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 : tim 1.2 */
142 : jsr166 1.29 boolean offer(E e);
143 : tim 1.1
144 :     /**
145 : jsr166 1.29 * Retrieves and removes the head of this queue. This method differs
146 : jsr166 1.45 * from {@link #poll() poll()} only in that it throws an exception if
147 :     * this queue is empty.
148 : tim 1.2 *
149 : jsr166 1.29 * @return the head of this queue
150 :     * @throws NoSuchElementException if this queue is empty
151 : tim 1.2 */
152 : tim 1.9 E remove();
153 : tim 1.1
154 :     /**
155 : jsr166 1.29 * Retrieves and removes the head of this queue,
156 : jsr166 1.38 * or returns {@code null} if this queue is empty.
157 : tim 1.2 *
158 : jsr166 1.38 * @return the head of this queue, or {@code null} if this queue is empty
159 : tim 1.2 */
160 : jsr166 1.29 E poll();
161 : tim 1.1
162 :     /**
163 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
164 : jsr166 1.33 * differs from {@link #peek peek} only in that it throws an exception
165 :     * if this queue is empty.
166 : tim 1.2 *
167 : jsr166 1.29 * @return the head of this queue
168 :     * @throws NoSuchElementException if this queue is empty
169 : tim 1.2 */
170 : tim 1.9 E element();
171 : jsr166 1.29
172 :     /**
173 :     * Retrieves, but does not remove, the head of this queue,
174 : jsr166 1.38 * or returns {@code null} if this queue is empty.
175 : jsr166 1.29 *
176 : jsr166 1.38 * @return the head of this queue, or {@code null} if this queue is empty
177 : jsr166 1.29 */
178 :     E peek();
179 : tim 1.1 }

Doug Lea
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