/*
 * 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 java.io.Serializable;

import com.google.common.annotations.GwtCompatible;

/**
 * An abstract base class for implementing the
 * <a href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator
 * pattern</a>. The {@link #delegate()} method must be overridden to return the
 * instance being decorated.
 *
 * <p>
 * This class does <i>not</i> 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.
 *
 * <p>
 * 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 (imported from Google Collections Library)
 */
@GwtCompatible
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. */
}
