1/*
2 * Copyright (C) 2007 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.google.common.collect;
18
19import com.google.common.annotations.Beta;
20import com.google.common.annotations.GwtCompatible;
21
22import java.util.NoSuchElementException;
23import java.util.Queue;
24
25/**
26 * A queue which forwards all its method calls to another queue. Subclasses
27 * should override one or more methods to modify the behavior of the backing
28 * queue as desired per the <a
29 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
30 *
31 * <p><b>Warning:</b> The methods of {@code ForwardingQueue} forward
32 * <b>indiscriminately</b> to the methods of the delegate. For example,
33 * overriding {@link #add} alone <b>will not</b> change the behavior of {@link
34 * #offer} which can lead to unexpected behavior. In this case, you should
35 * override {@code offer} as well, either providing your own implementation, or
36 * delegating to the provided {@code standardOffer} method.
37 *
38 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even
39 * when all of the methods that they depend on are thread-safe.
40 *
41 * @author Mike Bostock
42 * @author Louis Wasserman
43 * @since 2.0 (imported from Google Collections Library)
44 */
45@GwtCompatible
46public abstract class ForwardingQueue<E> extends ForwardingCollection<E>
47    implements Queue<E> {
48
49  /** Constructor for use by subclasses. */
50  protected ForwardingQueue() {}
51
52  @Override protected abstract Queue<E> delegate();
53
54  @Override
55  public boolean offer(E o) {
56    return delegate().offer(o);
57  }
58
59  @Override
60  public E poll() {
61    return delegate().poll();
62  }
63
64  @Override
65  public E remove() {
66    return delegate().remove();
67  }
68
69  @Override
70  public E peek() {
71    return delegate().peek();
72  }
73
74  @Override
75  public E element() {
76    return delegate().element();
77  }
78
79  /**
80   * A sensible definition of {@link #offer} in terms of {@link #add}. If you
81   * override {@link #add}, you may wish to override {@link #offer} to forward
82   * to this implementation.
83   *
84   * @since 7.0
85   */
86  @Beta protected boolean standardOffer(E e) {
87    try {
88      return add(e);
89    } catch (IllegalStateException caught) {
90      return false;
91    }
92  }
93
94  /**
95   * A sensible definition of {@link #peek} in terms of {@link #element}. If you
96   * override {@link #element}, you may wish to override {@link #peek} to
97   * forward to this implementation.
98   *
99   * @since 7.0
100   */
101  @Beta protected E standardPeek() {
102    try {
103      return element();
104    } catch (NoSuchElementException caught) {
105      return null;
106    }
107  }
108
109  /**
110   * A sensible definition of {@link #poll} in terms of {@link #remove}. If you
111   * override {@link #remove}, you may wish to override {@link #poll} to forward
112   * to this implementation.
113   *
114   * @since 7.0
115   */
116  @Beta protected E standardPoll() {
117    try {
118      return remove();
119    } catch (NoSuchElementException caught) {
120      return null;
121    }
122  }
123}
124