Queue.java revision 4fefecee9d4a5d2a4510f516b4015607b19e8d09 
1/*
2 *  Licensed to the Apache Software Foundation (ASF) under one or more
3 *  contributor license agreements.  See the NOTICE file distributed with
4 *  this work for additional information regarding copyright ownership.
5 *  The ASF licenses this file to You under the Apache License, Version 2.0
6 *  (the "License"); you may not use this file except in compliance with
7 *  the License.  You may obtain a copy of the License at
8 *
9 *     http://www.apache.org/licenses/LICENSE-2.0
10 *
11 *  Unless required by applicable law or agreed to in writing, software
12 *  distributed under the License is distributed on an "AS IS" BASIS,
13 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 *  See the License for the specific language governing permissions and
15 *  limitations under the License.
16 */
17package java.util;
18
19/**
20 * This kind of collection provides advanced operations compared to basic
21 * collections, such as insertion, extraction, and inspection.
22 * <p>
23 * Generally, a queue orders its elements by means of first-in-first-out.
24 * However, a priority queue orders its elements according to a comparator
25 * specified or the elements' natural order. Furthermore, a stack orders its
26 * elements last-in-first out.
27 * <p>
28 * A typical queue does not allow {@code null} to be inserted as its element,
29 * while some implementations such as {@code LinkedList} allow it. But {@code
30 * null} should not be inserted even in these implementations, since the method
31 * {@code poll} returns {@code null} to indicate that there is no element left
32 * in the queue.
33 * <p>
34 * {@code Queue} does not provide blocking queue methods, which would block
35 * until the operation of the method is allowed. See the
36 * {@link java.util.concurrent.BlockingQueue} interface for information about
37 * blocking queue methods.
38 */
39public interface Queue<E> extends Collection<E> {
40
41    /**
42     * Inserts the specified element into the queue provided that the condition
43     * allows such an operation. The method is generally preferable to
44     * {@link Collection#add}, since the latter might throw an exception if the
45     * operation fails.
46     *
47     * @param o
48     *            the specified element to insert into the queue.
49     * @return {@code true} if the operation succeeds and {@code false} if it
50     *         fails.
51     */
52    public boolean offer(E o);
53
54    /**
55     * Gets and removes the element at the head of the queue, or returns {@code
56     * null} if there is no element in the queue.
57     *
58     * @return the element at the head of the queue or {@code null} if there is
59     *         no element in the queue.
60     */
61    public E poll();
62
63    /**
64     * Gets and removes the element at the head of the queue. Throws a
65     * NoSuchElementException if there is no element in the queue.
66     *
67     * @return the element at the head of the queue.
68     * @throws NoSuchElementException
69     *             if there is no element in the queue.
70     */
71    public E remove();
72
73    /**
74     * Gets but does not remove the element at the head of the queue.
75     *
76     * @return the element at the head of the queue or {@code null} if there is
77     *         no element in the queue.
78     */
79    public E peek();
80
81    /**
82     * Gets but does not remove the element at the head of the queue. Throws a
83     * {@code NoSuchElementException} if there is no element in the queue.
84     *
85     * @return the element at the head of the queue.
86     * @throws NoSuchElementException
87     *             if there is no element in the queue.
88     */
89    public E element();
90
91}
92