ForwardingMapEntry.java revision 3c77433663281544363151bf284b0240dfd22a42 
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.Beta;
20import com.google.common.annotations.GwtCompatible;
21import com.google.common.base.Objects;
22
23import java.util.Map;
24import java.util.Map.Entry;
25
26import javax.annotation.Nullable;
27
28/**
29 * A map entry which forwards all its method calls to another map entry.
30 * Subclasses should override one or more methods to modify the behavior of the
31 * backing map entry as desired per the <a
32 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
33 *
34 * <p><i>Warning:</i> The methods of {@code ForwardingMapEntry} forward
35 * <i>indiscriminately</i> to the methods of the delegate. For example,
36 * overriding {@link #getValue} alone <i>will not</i> change the behavior of
37 * {@link #equals}, which can lead to unexpected behavior. In this case, you
38 * should override {@code equals} as well, either providing your own
39 * implementation, or delegating to the provided {@code standardEquals} method.
40 *
41 * <p>Each of the {@code standard} methods, where appropriate, use {@link
42 * Objects#equal} to test equality for both keys and values. This may not be
43 * the desired behavior for map implementations that use non-standard notions of
44 * key equality, such as the entry of a {@code SortedMap} whose comparator is
45 * not consistent with {@code equals}.
46 *
47 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even
48 * when all of the methods that they depend on are thread-safe.
49 *
50 * @author Mike Bostock
51 * @author Louis Wasserman
52 * @since 2.0 (imported from Google Collections Library)
53 */
54@GwtCompatible
55public abstract class ForwardingMapEntry<K, V>
56    extends ForwardingObject implements Map.Entry<K, V> {
57  // TODO(user): identify places where thread safety is actually lost
58
59  /** Constructor for use by subclasses. */
60  protected ForwardingMapEntry() {}
61
62  @Override protected abstract Map.Entry<K, V> delegate();
63
64  @Override
65  public K getKey() {
66    return delegate().getKey();
67  }
68
69  @Override
70  public V getValue() {
71    return delegate().getValue();
72  }
73
74  @Override
75  public V setValue(V value) {
76    return delegate().setValue(value);
77  }
78
79  @Override public boolean equals(@Nullable Object object) {
80    return delegate().equals(object);
81  }
82
83  @Override public int hashCode() {
84    return delegate().hashCode();
85  }
86
87  /**
88   * A sensible definition of {@link #equals(Object)} in terms of {@link
89   * #getKey()} and {@link #getValue()}. If you override either of these
90   * methods, you may wish to override {@link #equals(Object)} to forward to
91   * this implementation.
92   *
93   * @since 7.0
94   */
95  protected boolean standardEquals(@Nullable Object object) {
96    if (object instanceof Entry) {
97      Entry<?, ?> that = (Entry<?, ?>) object;
98      return Objects.equal(this.getKey(), that.getKey())
99          && Objects.equal(this.getValue(), that.getValue());
100    }
101    return false;
102  }
103
104  /**
105   * A sensible definition of {@link #hashCode()} in terms of {@link #getKey()}
106   * and {@link #getValue()}. If you override either of these methods, you may
107   * wish to override {@link #hashCode()} to forward to this implementation.
108   *
109   * @since 7.0
110   */
111  protected int standardHashCode() {
112    K k = getKey();
113    V v = getValue();
114    return ((k == null) ? 0 : k.hashCode()) ^ ((v == null) ? 0 : v.hashCode());
115  }
116
117  /**
118   * A sensible definition of {@link #toString} in terms of {@link
119   * #getKey} and {@link #getValue}. If you override either of these
120   * methods, you may wish to override {@link #equals} to forward to this
121   * implementation.
122   *
123   * @since 7.0
124   */
125  @Beta protected String standardToString() {
126    return getKey() + "=" + getValue();
127  }
128}
129