1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
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18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util;
37
38 /**
39 * A collection designed for holding elements prior to processing.
40 * Besides basic {@link Collection} operations, queues provide
41 * additional insertion, extraction, and inspection operations.
42 * Each of these methods exists in two forms: one throws an exception
43 * if the operation fails, the other returns a special value (either
44 * {@code null} or {@code false}, depending on the operation). The
45 * latter form of the insert operation is designed specifically for
46 * use with capacity-restricted {@code Queue} implementations; in most
47 * implementations, insert operations cannot fail.
48 *
49 * <table class="plain">
50 * <caption>Summary of Queue methods</caption>
51 * <tr>
52 * <td></td>
53 * <td style="text-align:center"><em>Throws exception</em></td>
54 * <td style="text-align:center"><em>Returns special value</em></td>
55 * </tr>
56 * <tr>
57 * <td><b>Insert</b></td>
58 * <td>{@link #add(Object) add(e)}</td>
59 * <td>{@link #offer(Object) offer(e)}</td>
60 * </tr>
61 * <tr>
62 * <td><b>Remove</b></td>
63 * <td>{@link #remove() remove()}</td>
64 * <td>{@link #poll() poll()}</td>
65 * </tr>
66 * <tr>
67 * <td><b>Examine</b></td>
68 * <td>{@link #element() element()}</td>
69 * <td>{@link #peek() peek()}</td>
70 * </tr>
71 * </table>
72 *
73 * <p>Queues typically, but do not necessarily, order elements in a
74 * FIFO (first-in-first-out) manner. Among the exceptions are
75 * priority queues, which order elements according to a supplied
76 * comparator, or the elements' natural ordering, and LIFO queues (or
77 * stacks) which order the elements LIFO (last-in-first-out).
78 * Whatever the ordering used, the <em>head</em> of the queue is that
79 * element which would be removed by a call to {@link #remove()} or
80 * {@link #poll()}. In a FIFO queue, all new elements are inserted at
81 * the <em>tail</em> of the queue. Other kinds of queues may use
82 * different placement rules. Every {@code Queue} implementation
83 * must specify its ordering properties.
84 *
85 * <p>The {@link #offer offer} method inserts an element if possible,
86 * otherwise returning {@code false}. This differs from the {@link
87 * java.util.Collection#add Collection.add} method, which can fail to
88 * add an element only by throwing an unchecked exception. The
89 * {@code offer} method is designed for use when failure is a normal,
90 * rather than exceptional occurrence, for example, in fixed-capacity
91 * (or "bounded") queues.
92 *
93 * <p>The {@link #remove()} and {@link #poll()} methods remove and
94 * return the head of the queue.
95 * Exactly which element is removed from the queue is a
96 * function of the queue's ordering policy, which differs from
97 * implementation to implementation. The {@code remove()} and
98 * {@code poll()} methods differ only in their behavior when the
99 * queue is empty: the {@code remove()} method throws an exception,
100 * while the {@code poll()} method returns {@code null}.
101 *
102 * <p>The {@link #element()} and {@link #peek()} methods return, but do
103 * not remove, the head of the queue.
104 *
105 * <p>The {@code Queue} interface does not define the <i>blocking queue
106 * methods</i>, which are common in concurrent programming. These methods,
107 * which wait for elements to appear or for space to become available, are
108 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
109 * extends this interface.
110 *
111 * <p>{@code Queue} implementations generally do not allow insertion
112 * of {@code null} elements, although some implementations, such as
113 * {@link LinkedList}, do not prohibit insertion of {@code null}.
114 * Even in the implementations that permit it, {@code null} should
115 * not be inserted into a {@code Queue}, as {@code null} is also
116 * used as a special return value by the {@code poll} method to
117 * indicate that the queue contains no elements.
118 *
119 * <p>{@code Queue} implementations generally do not define
120 * element-based versions of methods {@code equals} and
121 * {@code hashCode} but instead inherit the identity based versions
122 * from class {@code Object}, because element-based equality is not
123 * always well-defined for queues with the same elements but different
124 * ordering properties.
125 *
126 * <p>This interface is a member of the
127 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
128 * Java Collections Framework</a>.
129 *
130 * @since 1.5
131 * @author Doug Lea
132 * @param <E> the type of elements held in this queue
133 */
134 public interface Queue<E> extends Collection<E> {
135 /**
136 * Inserts the specified element into this queue if it is possible to do so
137 * immediately without violating capacity restrictions, returning
138 * {@code true} upon success and throwing an {@code IllegalStateException}
139 * if no space is currently available.
140 *
141 * @param e the element to add
142 * @return {@code true} (as specified by {@link Collection#add})
143 * @throws IllegalStateException if the element cannot be added at this
144 * time due to capacity restrictions
145 * @throws ClassCastException if the class of the specified element
146 * prevents it from being added to this queue
147 * @throws NullPointerException if the specified element is null and
148 * this queue does not permit null elements
149 * @throws IllegalArgumentException if some property of this element
150 * prevents it from being added to this queue
151 */
152 boolean add(E e);
153
154 /**
155 * Inserts the specified element into this queue if it is possible to do
156 * so immediately without violating capacity restrictions.
157 * When using a capacity-restricted queue, this method is generally
158 * preferable to {@link #add}, which can fail to insert an element only
159 * by throwing an exception.
160 *
161 * @param e the element to add
162 * @return {@code true} if the element was added to this queue, else
163 * {@code false}
164 * @throws ClassCastException if the class of the specified element
165 * prevents it from being added to this queue
166 * @throws NullPointerException if the specified element is null and
167 * this queue does not permit null elements
168 * @throws IllegalArgumentException if some property of this element
169 * prevents it from being added to this queue
170 */
171 boolean offer(E e);
172
173 /**
174 * Retrieves and removes the head of this queue. This method differs
175 * from {@link #poll() poll()} only in that it throws an exception if
176 * this queue is empty.
177 *
178 * @return the head of this queue
179 * @throws NoSuchElementException if this queue is empty
180 */
181 E remove();
182
183 /**
184 * Retrieves and removes the head of this queue,
185 * or returns {@code null} if this queue is empty.
186 *
187 * @return the head of this queue, or {@code null} if this queue is empty
188 */
189 E poll();
190
191 /**
192 * Retrieves, but does not remove, the head of this queue. This method
193 * differs from {@link #peek peek} only in that it throws an exception
194 * if this queue is empty.
195 *
196 * @return the head of this queue
197 * @throws NoSuchElementException if this queue is empty
198 */
199 E element();
200
201 /**
202 * Retrieves, but does not remove, the head of this queue,
203 * or returns {@code null} if this queue is empty.
204 *
205 * @return the head of this queue, or {@code null} if this queue is empty
206 */
207 E peek();
208 }