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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 : dl 1.24 * 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 : jsr166 1.38 * value (either {@code null} or {@code false}, depending on the
16 : dl 1.24 * operation). The latter form of the insert operation is designed
17 : jsr166 1.38 * specifically for use with capacity-restricted {@code Queue}
18 : dl 1.24 * implementations; in most implementations, insert operations cannot
19 :     * fail.
20 : dl 1.23 *
21 : jsr166 1.28 * <p>
22 :     * <table BORDER CELLPADDING=3 CELLSPACING=1>
23 : jsr166 1.39 * <caption>Summary of Queue methods</caption>
24 : dl 1.23 * <tr>
25 : dl 1.24 * <td></td>
26 :     * <td ALIGN=CENTER><em>Throws exception</em></td>
27 : jsr166 1.32 * <td ALIGN=CENTER><em>Returns special value</em></td>
28 : dl 1.23 * </tr>
29 :     * <tr>
30 : dl 1.24 * <td><b>Insert</b></td>
31 :     * <td>{@link #add add(e)}</td>
32 :     * <td>{@link #offer offer(e)}</td>
33 : dl 1.23 * </tr>
34 :     * <tr>
35 : dl 1.24 * <td><b>Remove</b></td>
36 :     * <td>{@link #remove remove()}</td>
37 :     * <td>{@link #poll poll()}</td>
38 : dl 1.23 * </tr>
39 :     * <tr>
40 : dl 1.24 * <td><b>Examine</b></td>
41 :     * <td>{@link #element element()}</td>
42 :     * <td>{@link #peek peek()}</td>
43 : dl 1.23 * </tr>
44 : dl 1.24 * </table>
45 : dholmes 1.8 *
46 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
47 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
48 :     * priority queues, which order elements according to a supplied
49 : dholmes 1.8 * comparator, or the elements' natural ordering, and LIFO queues (or
50 :     * stacks) which order the elements LIFO (last-in-first-out).
51 : dl 1.17 * 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 : jsr166 1.37 * the <em>tail</em> of the queue. Other kinds of queues may use
55 : jsr166 1.38 * different placement rules. Every {@code Queue} implementation
56 : dl 1.17 * must specify its ordering properties.
57 :     *
58 :     * <p>The {@link #offer offer} method inserts an element if possible,
59 : jsr166 1.38 * otherwise returning {@code false}. This differs from the {@link
60 : dl 1.17 * java.util.Collection#add Collection.add} method, which can fail to
61 :     * add an element only by throwing an unchecked exception. The
62 : jsr166 1.38 * {@code offer} method is designed for use when failure is a normal,
63 : dl 1.17 * rather than exceptional occurrence, for example, in fixed-capacity
64 :     * (or &quot;bounded&quot;) queues.
65 : tim 1.9 *
66 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
67 : dholmes 1.8 * return the head of the queue.
68 :     * Exactly which element is removed from the queue is a
69 : dl 1.7 * function of the queue's ordering policy, which differs from
70 : jsr166 1.38 * 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 : tim 1.1 *
75 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
76 : dholmes 1.10 * not remove, the head of the queue.
77 : tim 1.1 *
78 : jsr166 1.38 * <p>The {@code Queue} interface does not define the <i>blocking queue
79 : tim 1.2 * 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 : jsr166 1.38 * <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 : dl 1.7 * indicate that the queue contains no elements.
91 : tim 1.2 *
92 : jsr166 1.38 * <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 : dl 1.16 * always well-defined for queues with the same elements but different
97 :     * ordering properties.
98 :     *
99 :     *
100 : tim 1.2 * <p>This interface is a member of the
101 : jsr166 1.35 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
102 : tim 1.2 * Java Collections Framework</a>.
103 :     *
104 : tim 1.11 * @see java.util.Collection
105 : tim 1.2 * @see LinkedList
106 :     * @see PriorityQueue
107 : dholmes 1.15 * @see java.util.concurrent.LinkedBlockingQueue
108 : tim 1.2 * @see java.util.concurrent.BlockingQueue
109 :     * @see java.util.concurrent.ArrayBlockingQueue
110 :     * @see java.util.concurrent.LinkedBlockingQueue
111 :     * @see java.util.concurrent.PriorityBlockingQueue
112 : dl 1.7 * @since 1.5
113 :     * @author Doug Lea
114 : dl 1.21 * @param <E> the type of elements held in this collection
115 : tim 1.2 */
116 : tim 1.1 public interface Queue<E> extends Collection<E> {
117 :     /**
118 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do so
119 :     * immediately without violating capacity restrictions, returning
120 : jsr166 1.38 * {@code true} upon success and throwing an {@code IllegalStateException}
121 : jsr166 1.29 * if no space is currently available.
122 :     *
123 :     * @param e the element to add
124 : jsr166 1.38 * @return {@code true} (as specified by {@link Collection#add})
125 : jsr166 1.29 * @throws IllegalStateException if the element cannot be added at this
126 :     * time due to capacity restrictions
127 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
128 :     * prevents it from being added to this queue
129 : jsr166 1.29 * @throws NullPointerException if the specified element is null and
130 : jsr166 1.34 * this queue does not permit null elements
131 : jsr166 1.29 * @throws IllegalArgumentException if some property of this element
132 :     * prevents it from being added to this queue
133 : tim 1.2 */
134 : jsr166 1.29 boolean add(E e);
135 : tim 1.1
136 :     /**
137 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do
138 :     * so immediately without violating capacity restrictions.
139 : jsr166 1.31 * When using a capacity-restricted queue, this method is generally
140 : jsr166 1.29 * preferable to {@link #add}, which can fail to insert an element only
141 :     * by throwing an exception.
142 : tim 1.2 *
143 : jsr166 1.29 * @param e the element to add
144 : jsr166 1.38 * @return {@code true} if the element was added to this queue, else
145 :     * {@code false}
146 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
147 :     * prevents it from being added to this queue
148 : jsr166 1.29 * @throws NullPointerException if the specified element is null and
149 :     * this queue does not permit null elements
150 :     * @throws IllegalArgumentException if some property of this element
151 :     * prevents it from being added to this queue
152 : tim 1.2 */
153 : jsr166 1.29 boolean offer(E e);
154 : tim 1.1
155 :     /**
156 : jsr166 1.29 * Retrieves and removes the head of this queue. This method differs
157 : jsr166 1.33 * from {@link #poll poll} only in that it throws an exception if this
158 :     * queue is empty.
159 : tim 1.2 *
160 : jsr166 1.29 * @return the head of this queue
161 :     * @throws NoSuchElementException if this queue is empty
162 : tim 1.2 */
163 : tim 1.9 E remove();
164 : tim 1.1
165 :     /**
166 : jsr166 1.29 * Retrieves and removes the head of this queue,
167 : jsr166 1.38 * or returns {@code null} if this queue is empty.
168 : tim 1.2 *
169 : jsr166 1.38 * @return the head of this queue, or {@code null} if this queue is empty
170 : tim 1.2 */
171 : jsr166 1.29 E poll();
172 : tim 1.1
173 :     /**
174 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
175 : jsr166 1.33 * differs from {@link #peek peek} only in that it throws an exception
176 :     * if this queue is empty.
177 : tim 1.2 *
178 : jsr166 1.29 * @return the head of this queue
179 :     * @throws NoSuchElementException if this queue is empty
180 : tim 1.2 */
181 : tim 1.9 E element();
182 : jsr166 1.29
183 :     /**
184 :     * Retrieves, but does not remove, the head of this queue,
185 : jsr166 1.38 * or returns {@code null} if this queue is empty.
186 : jsr166 1.29 *
187 : jsr166 1.38 * @return the head of this queue, or {@code null} if this queue is empty
188 : jsr166 1.29 */
189 :     E peek();
190 : tim 1.1 }

Doug Lea
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