/*
 * 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 decorator pattern.
Warning: The methods of ForwardingQueue forward indiscriminately to the methods of the delegate. For example, overriding ForwardingCollection.add alone will not change the behavior of offer which can lead to unexpected behavior. In this case, you should override offer as well, either providing your own implementation, or delegating to the provided standardOffer method. 
default method warning: This class does not forward calls to 
default methods. Instead, it inherits their default implementations. When those implementations invoke methods, they invoke methods on the ForwardingQueue. 
The 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, Louis Wasserman
Since:
2.0/**
 * 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><b>Warning:</b> The methods of {@code ForwardingQueue} forward <b>indiscriminately</b> to the
 * methods of the delegate. For example, overriding {@link #add} alone <b>will not</b> 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><b>{@code default} method warning:</b> This class does <i>not</i> 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<E> implements Queue<E> {

  
Constructor for use by subclasses. /** 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 offer in terms of ForwardingCollection.add. If you override ForwardingCollection.add, you may wish to override offer to forward to this implementation. 
Since:
7.0/**
   * 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 peek in terms of element. If you override element, you may wish to override peek to forward to this implementation. 
Since:
7.0/**
   * 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 poll in terms of remove. If you override remove, you may wish to override poll to forward to this implementation. 
Since:
7.0/**
   * 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;
    }
  }
}