com.google.common.collect.ForwardingObject Maven / Gradle / Ivy
/*
* 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 java.io.Serializable;
/**
* An abstract base class for implementing the decorator pattern. The {@link
* #delegate()} method must be overridden to return the instance being decorated.
*
* This class does not forward the {@code hashCode} and {@code equals} methods through to
* the backing object, but relies on {@code Object}'s implementation. This is necessary to preserve
* the symmetry of {@code equals}. Custom definitions of equality are usually based on an interface,
* such as {@code Set} or {@code List}, so that the implementation of {@code equals} can cast the
* object being tested for equality to the custom interface. {@code ForwardingObject} implements no
* such custom interfaces directly; they are implemented only in subclasses. Therefore, forwarding
* {@code equals} would break symmetry, as the forwarding object might consider itself equal to the
* object being tested, but the reverse could not be true. This behavior is consistent with the
* JDK's collection wrappers, such as {@link java.util.Collections#unmodifiableCollection}. Use an
* interface-specific subclass of {@code ForwardingObject}, such as {@link ForwardingList}, to
* preserve equality behavior, or override {@code equals} directly.
*
*
The {@code toString} method is forwarded to the delegate. Although this class does not
* implement {@link Serializable}, a serializable subclass may be created since this class has a
* parameter-less constructor.
*
* @author Mike Bostock
* @since 2.0
*/
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class ForwardingObject {
/** Constructor for use by subclasses. */
protected ForwardingObject() {}
/**
* Returns the backing delegate instance that methods are forwarded to. Abstract subclasses
* generally override this method with an abstract method that has a more specific return type,
* such as {@link ForwardingSet#delegate}. Concrete subclasses override this method to supply the
* instance being decorated.
*/
protected abstract Object delegate();
/** Returns the string representation generated by the delegate's {@code toString} method. */
@Override
public String toString() {
return delegate().toString();
}
/* No equals or hashCode. See class comments for details. */
}