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Java example source code file (ForwardingQueue.java)

This example Java source code file (ForwardingQueue.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

canignorereturnvalue, consider, forwardingcollection, forwardingqueue, gwtcompatible, illegalstateexception, nosuchelementexception, override, queue, todo, util

The ForwardingQueue.java Java example source code

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;

import java.util.NoSuchElementException;
import java.util.Queue;

/**
 * A queue which forwards all its method calls to another queue. Subclasses
 * should override one or more methods to modify the behavior of the backing
 * queue as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p>Warning: The methods of {@code ForwardingQueue} forward
 * <b>indiscriminately to the methods of the delegate. For example,
 * overriding {@link #add} alone <b>will not change the behavior of {@link
 * #offer} which can lead to unexpected behavior. In this case, you should
 * override {@code offer} as well, either providing your own implementation, or
 * delegating to the provided {@code standardOffer} method.
 *
 * <p>{@code default} method warning: This class does not forward calls to {@code
 * default} methods. Instead, it inherits their default implementations. When those implementations
 * invoke methods, they invoke methods on the {@code ForwardingQueue}.
 *
 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even
 * when all of the methods that they depend on are thread-safe.
 *
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
public abstract class ForwardingQueue<E> extends ForwardingCollection implements Queue {

  /** Constructor for use by subclasses. */
  protected ForwardingQueue() {}

  @Override
  protected abstract Queue<E> delegate();

  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  @Override
  public boolean offer(E o) {
    return delegate().offer(o);
  }

  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  @Override
  public E poll() {
    return delegate().poll();
  }

  @CanIgnoreReturnValue
  @Override
  public E remove() {
    return delegate().remove();
  }

  @Override
  public E peek() {
    return delegate().peek();
  }

  @Override
  public E element() {
    return delegate().element();
  }

  /**
   * A sensible definition of {@link #offer} in terms of {@link #add}. If you
   * override {@link #add}, you may wish to override {@link #offer} to forward
   * to this implementation.
   *
   * @since 7.0
   */
  protected boolean standardOffer(E e) {
    try {
      return add(e);
    } catch (IllegalStateException caught) {
      return false;
    }
  }

  /**
   * A sensible definition of {@link #peek} in terms of {@link #element}. If you
   * override {@link #element}, you may wish to override {@link #peek} to
   * forward to this implementation.
   *
   * @since 7.0
   */
  protected E standardPeek() {
    try {
      return element();
    } catch (NoSuchElementException caught) {
      return null;
    }
  }

  /**
   * A sensible definition of {@link #poll} in terms of {@link #remove}. If you
   * override {@link #remove}, you may wish to override {@link #poll} to forward
   * to this implementation.
   *
   * @since 7.0
   */
  protected E standardPoll() {
    try {
      return remove();
    } catch (NoSuchElementException caught) {
      return null;
    }
  }
}

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