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;
020
021import java.io.Serializable;
022
023/**
024 * An abstract base class for implementing the <a
025 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
026 * The {@link #delegate()} method must be overridden to return the instance
027 * being decorated.
028 *
029 * <p>This class does <i>not</i> forward the {@code hashCode} and {@code equals}
030 * methods through to the backing object, but relies on {@code Object}'s
031 * implementation. This is necessary to preserve the symmetry of {@code equals}.
032 * Custom definitions of equality are usually based on an interface, such as
033 * {@code Set} or {@code List}, so that the implementation of {@code equals} can
034 * cast the object being tested for equality to the custom interface. {@code
035 * ForwardingObject} implements no such custom interfaces directly; they
036 * are implemented only in subclasses. Therefore, forwarding {@code equals}
037 * would break symmetry, as the forwarding object might consider itself equal to
038 * the object being tested, but the reverse could not be true. This behavior is
039 * consistent with the JDK's collection wrappers, such as
040 * {@link java.util.Collections#unmodifiableCollection}. Use an
041 * interface-specific subclass of {@code ForwardingObject}, such as {@link
042 * ForwardingList}, to preserve equality behavior, or override {@code equals}
043 * directly.
044 *
045 * <p>The {@code toString} method is forwarded to the delegate. Although this
046 * class does not implement {@link Serializable}, a serializable subclass may be
047 * created since this class has a parameter-less constructor.
048 *
049 * @author Mike Bostock
050 * @since 2.0 (imported from Google Collections Library)
051 */
052@GwtCompatible
053public abstract class ForwardingObject {
054
055  /** Constructor for use by subclasses. */
056  protected ForwardingObject() {}
057
058  /**
059   * Returns the backing delegate instance that methods are forwarded to.
060   * Abstract subclasses generally override this method with an abstract method
061   * that has a more specific return type, such as {@link
062   * ForwardingSet#delegate}. Concrete subclasses override this method to supply
063   * the instance being decorated.
064   */
065  protected abstract Object delegate();
066
067  /**
068   * Returns the string representation generated by the delegate's
069   * {@code toString} method.
070   */
071  @Override public String toString() {
072    return delegate().toString();
073  }
074
075  /* No equals or hashCode. See class comments for details. */
076}