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
17 package com.google.common.collect;
18
19 import com.google.common.annotations.GwtCompatible;
20 import java.io.Serializable;
21
22 /**
23 * An abstract base class for implementing the <a
24 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>. The {@link
25 * #delegate()} method must be overridden to return the instance being decorated.
26 *
27 * <p>This class does <i>not</i> forward the {@code hashCode} and {@code equals} methods through to
28 * the backing object, but relies on {@code Object}'s implementation. This is necessary to preserve
29 * the symmetry of {@code equals}. Custom definitions of equality are usually based on an interface,
30 * such as {@code Set} or {@code List}, so that the implementation of {@code equals} can cast the
31 * object being tested for equality to the custom interface. {@code ForwardingObject} implements no
32 * such custom interfaces directly; they are implemented only in subclasses. Therefore, forwarding
33 * {@code equals} would break symmetry, as the forwarding object might consider itself equal to the
34 * object being tested, but the reverse could not be true. This behavior is consistent with the
35 * JDK's collection wrappers, such as {@link java.util.Collections#unmodifiableCollection}. Use an
36 * interface-specific subclass of {@code ForwardingObject}, such as {@link ForwardingList}, to
37 * preserve equality behavior, or override {@code equals} directly.
38 *
39 * <p>The {@code toString} method is forwarded to the delegate. Although this class does not
40 * implement {@link Serializable}, a serializable subclass may be created since this class has a
41 * parameter-less constructor.
42 *
43 * @author Mike Bostock
44 * @since 2.0
45 */
46 @GwtCompatible
47 public abstract class ForwardingObject {
48
49 /** Constructor for use by subclasses. */
50 protected ForwardingObject() {}
51
52 /**
53 * Returns the backing delegate instance that methods are forwarded to. Abstract subclasses
54 * generally override this method with an abstract method that has a more specific return type,
55 * such as {@link ForwardingSet#delegate}. Concrete subclasses override this method to supply the
56 * instance being decorated.
57 */
58 protected abstract Object delegate();
59
60 /** Returns the string representation generated by the delegate's {@code toString} method. */
61 @Override
62 public String toString() {
63 return delegate().toString();
64 }
65
66 /* No equals or hashCode. See class comments for details. */
67 }