001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtCompatible;
021
022import java.util.NoSuchElementException;
023import java.util.Queue;
024
025/**
026 * A queue which forwards all its method calls to another queue. Subclasses
027 * should override one or more methods to modify the behavior of the backing
028 * queue as desired per the <a
029 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
030 *
031 * <p><b>Warning:</b> The methods of {@code ForwardingQueue} forward
032 * <b>indiscriminately</b> to the methods of the delegate. For example,
033 * overriding {@link #add} alone <b>will not</b> change the behavior of {@link
034 * #offer} which can lead to unexpected behavior. In this case, you should
035 * override {@code offer} as well, either providing your own implementation, or
036 * delegating to the provided {@code standardOffer} method.
037 *
038 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even
039 * when all of the methods that they depend on are thread-safe.
040 *
041 * @author Mike Bostock
042 * @author Louis Wasserman
043 * @since 2.0 (imported from Google Collections Library)
044 */
045@GwtCompatible
046public abstract class ForwardingQueue<E> extends ForwardingCollection<E>
047    implements Queue<E> {
048
049  /** Constructor for use by subclasses. */
050  protected ForwardingQueue() {}
051
052  @Override protected abstract Queue<E> delegate();
053
054  @Override
055  public boolean offer(E o) {
056    return delegate().offer(o);
057  }
058
059  @Override
060  public E poll() {
061    return delegate().poll();
062  }
063
064  @Override
065  public E remove() {
066    return delegate().remove();
067  }
068
069  @Override
070  public E peek() {
071    return delegate().peek();
072  }
073
074  @Override
075  public E element() {
076    return delegate().element();
077  }
078
079  /**
080   * A sensible definition of {@link #offer} in terms of {@link #add}. If you
081   * override {@link #add}, you may wish to override {@link #offer} to forward
082   * to this implementation.
083   * 
084   * @since 7.0
085   */
086  @Beta protected boolean standardOffer(E e) {
087    try {
088      return add(e);
089    } catch (IllegalStateException caught) {
090      return false;
091    }
092  }
093
094  /**
095   * A sensible definition of {@link #peek} in terms of {@link #element}. If you
096   * override {@link #element}, you may wish to override {@link #peek} to
097   * forward to this implementation.
098   * 
099   * @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}