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