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.GwtCompatible;
020import com.google.errorprone.annotations.CanIgnoreReturnValue;
021import java.util.NoSuchElementException;
022import java.util.Queue;
023import org.jspecify.annotations.Nullable;
024
025/**
026 * A queue which forwards all its method calls to another queue. Subclasses should override one or
027 * more methods to modify the behavior of the backing queue as desired per the <a
028 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
029 *
030 * <p><b>Warning:</b> The methods of {@code ForwardingQueue} forward <b>indiscriminately</b> to the
031 * methods of the delegate. For example, overriding {@link #add} alone <b>will not</b> change the
032 * behavior of {@link #offer} which can lead to unexpected behavior. In this case, you should
033 * override {@code offer} as well, either providing your own implementation, or delegating to the
034 * provided {@code standardOffer} method.
035 *
036 * <p><b>{@code default} method warning:</b> This class does <i>not</i> forward calls to {@code
037 * default} methods. Instead, it inherits their default implementations. When those implementations
038 * invoke methods, they invoke methods on the {@code ForwardingQueue}.
039 *
040 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even when all of the
041 * methods that they depend on are thread-safe.
042 *
043 * @author Mike Bostock
044 * @author Louis Wasserman
045 * @since 2.0
046 */
047@GwtCompatible
048public abstract class ForwardingQueue<E extends @Nullable Object> extends ForwardingCollection<E>
049    implements Queue<E> {
050
051  /** Constructor for use by subclasses. */
052  protected ForwardingQueue() {}
053
054  @Override
055  protected abstract Queue<E> delegate();
056
057  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
058  @Override
059  public boolean offer(@ParametricNullness E o) {
060    return delegate().offer(o);
061  }
062
063  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
064  @Override
065  public @Nullable E poll() {
066    return delegate().poll();
067  }
068
069  @CanIgnoreReturnValue
070  @Override
071  @ParametricNullness
072  public E remove() {
073    return delegate().remove();
074  }
075
076  @Override
077  public @Nullable E peek() {
078    return delegate().peek();
079  }
080
081  @Override
082  @ParametricNullness
083  public E element() {
084    return delegate().element();
085  }
086
087  /**
088   * A sensible definition of {@link #offer} in terms of {@link #add}. If you override {@link #add},
089   * you may wish to override {@link #offer} to forward to this implementation.
090   *
091   * @since 7.0
092   */
093  protected boolean standardOffer(@ParametricNullness E e) {
094    try {
095      return add(e);
096    } catch (IllegalStateException caught) {
097      return false;
098    }
099  }
100
101  /**
102   * A sensible definition of {@link #peek} in terms of {@link #element}. If you override {@link
103   * #element}, you may wish to override {@link #peek} to forward to this implementation.
104   *
105   * @since 7.0
106   */
107  protected @Nullable E standardPeek() {
108    try {
109      return element();
110    } catch (NoSuchElementException caught) {
111      return null;
112    }
113  }
114
115  /**
116   * A sensible definition of {@link #poll} in terms of {@link #remove}. If you override {@link
117   * #remove}, you may wish to override {@link #poll} to forward to this implementation.
118   *
119   * @since 7.0
120   */
121  protected @Nullable E standardPoll() {
122    try {
123      return remove();
124    } catch (NoSuchElementException caught) {
125      return null;
126    }
127  }
128}