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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 * <table BORDER CELLPADDING=3 CELLSPACING=1>
22 : jsr166 1.39 * <caption>Summary of Queue methods</caption>
23 : dl 1.23 * <tr>
24 : dl 1.24 * <td></td>
25 :     * <td ALIGN=CENTER><em>Throws exception</em></td>
26 : jsr166 1.32 * <td ALIGN=CENTER><em>Returns special value</em></td>
27 : dl 1.23 * </tr>
28 :     * <tr>
29 : dl 1.24 * <td><b>Insert</b></td>
30 : dl 1.40 * <td>{@link Queue#add add(e)}</td>
31 :     * <td>{@link Queue#offer offer(e)}</td>
32 : dl 1.23 * </tr>
33 :     * <tr>
34 : dl 1.24 * <td><b>Remove</b></td>
35 : dl 1.40 * <td>{@link Queue#remove remove()}</td>
36 :     * <td>{@link Queue#poll poll()}</td>
37 : dl 1.23 * </tr>
38 :     * <tr>
39 : dl 1.24 * <td><b>Examine</b></td>
40 : dl 1.40 * <td>{@link Queue#element element()}</td>
41 :     * <td>{@link Queue#peek peek()}</td>
42 : dl 1.23 * </tr>
43 : dl 1.24 * </table>
44 : dholmes 1.8 *
45 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
46 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
47 :     * priority queues, which order elements according to a supplied
48 : dholmes 1.8 * comparator, or the elements' natural ordering, and LIFO queues (or
49 :     * stacks) which order the elements LIFO (last-in-first-out).
50 : dl 1.17 * 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 : jsr166 1.37 * the <em>tail</em> of the queue. Other kinds of queues may use
54 : jsr166 1.38 * different placement rules. Every {@code Queue} implementation
55 : dl 1.17 * must specify its ordering properties.
56 :     *
57 :     * <p>The {@link #offer offer} method inserts an element if possible,
58 : jsr166 1.38 * otherwise returning {@code false}. This differs from the {@link
59 : dl 1.17 * java.util.Collection#add Collection.add} method, which can fail to
60 :     * add an element only by throwing an unchecked exception. The
61 : jsr166 1.38 * {@code offer} method is designed for use when failure is a normal,
62 : dl 1.17 * rather than exceptional occurrence, for example, in fixed-capacity
63 :     * (or &quot;bounded&quot;) queues.
64 : tim 1.9 *
65 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
66 : dholmes 1.8 * return the head of the queue.
67 :     * Exactly which element is removed from the queue is a
68 : dl 1.7 * function of the queue's ordering policy, which differs from
69 : jsr166 1.38 * 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 : tim 1.1 *
74 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
75 : dholmes 1.10 * not remove, the head of the queue.
76 : tim 1.1 *
77 : jsr166 1.38 * <p>The {@code Queue} interface does not define the <i>blocking queue
78 : tim 1.2 * 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 : jsr166 1.38 * <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 : dl 1.7 * indicate that the queue contains no elements.
90 : tim 1.2 *
91 : jsr166 1.38 * <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 : dl 1.16 * always well-defined for queues with the same elements but different
96 :     * ordering properties.
97 :     *
98 : tim 1.2 * <p>This interface is a member of the
99 : jsr166 1.35 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
100 : tim 1.2 * Java Collections Framework</a>.
101 :     *
102 : dl 1.7 * @since 1.5
103 :     * @author Doug Lea
104 : jsr166 1.42 * @param <E> the type of elements held in this queue
105 : tim 1.2 */
106 : tim 1.1 public interface Queue<E> extends Collection<E> {
107 :     /**
108 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do so
109 :     * immediately without violating capacity restrictions, returning
110 : jsr166 1.38 * {@code true} upon success and throwing an {@code IllegalStateException}
111 : jsr166 1.29 * if no space is currently available.
112 :     *
113 :     * @param e the element to add
114 : jsr166 1.38 * @return {@code true} (as specified by {@link Collection#add})
115 : jsr166 1.29 * @throws IllegalStateException if the element cannot be added at this
116 :     * time due to capacity restrictions
117 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
118 :     * prevents it from being added to this queue
119 : jsr166 1.29 * @throws NullPointerException if the specified element is null and
120 : jsr166 1.34 * this queue does not permit null elements
121 : jsr166 1.29 * @throws IllegalArgumentException if some property of this element
122 :     * prevents it from being added to this queue
123 : tim 1.2 */
124 : jsr166 1.29 boolean add(E e);
125 : tim 1.1
126 :     /**
127 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do
128 :     * so immediately without violating capacity restrictions.
129 : jsr166 1.31 * When using a capacity-restricted queue, this method is generally
130 : jsr166 1.29 * preferable to {@link #add}, which can fail to insert an element only
131 :     * by throwing an exception.
132 : tim 1.2 *
133 : jsr166 1.29 * @param e the element to add
134 : jsr166 1.38 * @return {@code true} if the element was added to this queue, else
135 :     * {@code false}
136 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
137 :     * prevents it from being added to this queue
138 : jsr166 1.29 * @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 : tim 1.2 */
143 : jsr166 1.29 boolean offer(E e);
144 : tim 1.1
145 :     /**
146 : jsr166 1.29 * Retrieves and removes the head of this queue. This method differs
147 : jsr166 1.33 * from {@link #poll poll} only in that it throws an exception if this
148 :     * queue is empty.
149 : tim 1.2 *
150 : jsr166 1.29 * @return the head of this queue
151 :     * @throws NoSuchElementException if this queue is empty
152 : tim 1.2 */
153 : tim 1.9 E remove();
154 : tim 1.1
155 :     /**
156 : jsr166 1.29 * Retrieves and removes the head of this queue,
157 : jsr166 1.38 * or returns {@code null} if this queue is empty.
158 : tim 1.2 *
159 : jsr166 1.38 * @return the head of this queue, or {@code null} if this queue is empty
160 : tim 1.2 */
161 : jsr166 1.29 E poll();
162 : tim 1.1
163 :     /**
164 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
165 : jsr166 1.33 * differs from {@link #peek peek} only in that it throws an exception
166 :     * if this queue is empty.
167 : tim 1.2 *
168 : jsr166 1.29 * @return the head of this queue
169 :     * @throws NoSuchElementException if this queue is empty
170 : tim 1.2 */
171 : tim 1.9 E element();
172 : jsr166 1.29
173 :     /**
174 :     * Retrieves, but does not remove, the head of this queue,
175 : jsr166 1.38 * or returns {@code null} if this queue is empty.
176 : jsr166 1.29 *
177 : jsr166 1.38 * @return the head of this queue, or {@code null} if this queue is empty
178 : jsr166 1.29 */
179 :     E peek();
180 : tim 1.1 }

Doug Lea
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