1/*
2 * Copyright (C) 2007 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.google.common.collect;
18
19import com.google.common.annotations.GwtCompatible;
20
21import java.io.Serializable;
22
23/**
24 * An abstract base class for implementing the <a
25 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
26 * The {@link #delegate()} method must be overridden to return the instance
27 * being decorated.
28 *
29 * <p>This class does <i>not</i> forward the {@code hashCode} and {@code equals}
30 * methods through to the backing object, but relies on {@code Object}'s
31 * implementation. This is necessary to preserve the symmetry of {@code equals}.
32 * Custom definitions of equality are usually based on an interface, such as
33 * {@code Set} or {@code List}, so that the implementation of {@code equals} can
34 * cast the object being tested for equality to the custom interface. {@code
35 * ForwardingObject} implements no such custom interfaces directly; they
36 * are implemented only in subclasses. Therefore, forwarding {@code equals}
37 * would break symmetry, as the forwarding object might consider itself equal to
38 * the object being tested, but the reverse could not be true. This behavior is
39 * consistent with the JDK's collection wrappers, such as
40 * {@link java.util.Collections#unmodifiableCollection}. Use an
41 * interface-specific subclass of {@code ForwardingObject}, such as {@link
42 * ForwardingList}, to preserve equality behavior, or override {@code equals}
43 * directly.
44 *
45 * <p>The {@code toString} method is forwarded to the delegate. Although this
46 * class does not implement {@link Serializable}, a serializable subclass may be
47 * created since this class has a parameter-less constructor.
48 *
49 * @author Mike Bostock
50 * @since 2.0 (imported from Google Collections Library)
51 */
52@GwtCompatible
53public abstract class ForwardingObject {
54
55  /** Constructor for use by subclasses. */
56  protected ForwardingObject() {}
57
58  /**
59   * Returns the backing delegate instance that methods are forwarded to.
60   * Abstract subclasses generally override this method with an abstract method
61   * that has a more specific return type, such as {@link
62   * ForwardingSet#delegate}. Concrete subclasses override this method to supply
63   * the instance being decorated.
64   */
65  protected abstract Object delegate();
66
67  /**
68   * Returns the string representation generated by the delegate's
69   * {@code toString} method.
70   */
71  @Override public String toString() {
72    return delegate().toString();
73  }
74
75  /* No equals or hashCode. See class comments for details. */
76}
77