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 java.io.Serializable; 021 022/** 023 * An abstract base class for implementing the <a 024 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>. The {@link 025 * #delegate()} method must be overridden to return the instance being decorated. 026 * 027 * <p>This class does <i>not</i> forward the {@code hashCode} and {@code equals} methods through to 028 * the backing object, but relies on {@code Object}'s implementation. This is necessary to preserve 029 * the symmetry of {@code equals}. Custom definitions of equality are usually based on an interface, 030 * such as {@code Set} or {@code List}, so that the implementation of {@code equals} can cast the 031 * object being tested for equality to the custom interface. {@code ForwardingObject} implements no 032 * such custom interfaces directly; they are implemented only in subclasses. Therefore, forwarding 033 * {@code equals} would break symmetry, as the forwarding object might consider itself equal to the 034 * object being tested, but the reverse could not be true. This behavior is consistent with the 035 * JDK's collection wrappers, such as {@link java.util.Collections#unmodifiableCollection}. Use an 036 * interface-specific subclass of {@code ForwardingObject}, such as {@link ForwardingList}, to 037 * preserve equality behavior, or override {@code equals} directly. 038 * 039 * <p>The {@code toString} method is forwarded to the delegate. Although this class does not 040 * implement {@link Serializable}, a serializable subclass may be created since this class has a 041 * parameter-less constructor. 042 * 043 * @author Mike Bostock 044 * @since 2.0 045 */ 046@GwtCompatible 047@ElementTypesAreNonnullByDefault 048public abstract class ForwardingObject { 049 050 /** Constructor for use by subclasses. */ 051 protected ForwardingObject() {} 052 053 /** 054 * Returns the backing delegate instance that methods are forwarded to. Abstract subclasses 055 * generally override this method with an abstract method that has a more specific return type, 056 * such as {@link ForwardingSet#delegate}. Concrete subclasses override this method to supply the 057 * instance being decorated. 058 */ 059 protected abstract Object delegate(); 060 061 /** Returns the string representation generated by the delegate's {@code toString} method. */ 062 @Override 063 public String toString() { 064 return delegate().toString(); 065 } 066 067 /* No equals or hashCode. See class comments for details. */ 068}