guava/src/com/google/common/collect/ForwardingObject.java

/*
 * 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 <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
 */
@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. */
}