guava/src/com/google/common/collect/ForwardingMapEntry.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.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Objects;

import java.util.Map;
import java.util.Map.Entry;

import javax.annotation.Nullable;

/**
 * A map entry which forwards all its method calls to another map entry.
 * Subclasses should override one or more methods to modify the behavior of the
 * backing map entry as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><i>Warning:</i> The methods of {@code ForwardingMapEntry} forward
 * <i>indiscriminately</i> to the methods of the delegate. For example,
 * overriding {@link #getValue} alone <i>will not</i> change the behavior of
 * {@link #equals}, which can lead to unexpected behavior. In this case, you
 * should override {@code equals} as well, either providing your own
 * implementation, or delegating to the provided {@code standardEquals} method.
 *
 * <p>Each of the {@code standard} methods, where appropriate, use {@link
 * Objects#equal} to test equality for both keys and values. This may not be
 * the desired behavior for map implementations that use non-standard notions of
 * key equality, such as the entry of a {@code SortedMap} whose comparator is
 * not consistent with {@code equals}.
 *
 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even
 * when all of the methods that they depend on are thread-safe.
 *
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public abstract class ForwardingMapEntry<K, V>
    extends ForwardingObject implements Map.Entry<K, V> {
  // TODO(user): identify places where thread safety is actually lost

  /** Constructor for use by subclasses. */
  protected ForwardingMapEntry() {}

  @Override protected abstract Map.Entry<K, V> delegate();

  @Override
  public K getKey() {
    return delegate().getKey();
  }

  @Override
  public V getValue() {
    return delegate().getValue();
  }

  @Override
  public V setValue(V value) {
    return delegate().setValue(value);
  }

  @Override public boolean equals(@Nullable Object object) {
    return delegate().equals(object);
  }

  @Override public int hashCode() {
    return delegate().hashCode();
  }

  /**
   * A sensible definition of {@link #equals(Object)} in terms of {@link
   * #getKey()} and {@link #getValue()}. If you override either of these
   * methods, you may wish to override {@link #equals(Object)} to forward to
   * this implementation.
   *
   * @since 7.0
   */
  protected boolean standardEquals(@Nullable Object object) {
    if (object instanceof Entry) {
      Entry<?, ?> that = (Entry<?, ?>) object;
      return Objects.equal(this.getKey(), that.getKey())
          && Objects.equal(this.getValue(), that.getValue());
    }
    return false;
  }

  /**
   * A sensible definition of {@link #hashCode()} in terms of {@link #getKey()}
   * and {@link #getValue()}. If you override either of these methods, you may
   * wish to override {@link #hashCode()} to forward to this implementation.
   *
   * @since 7.0
   */
  protected int standardHashCode() {
    K k = getKey();
    V v = getValue();
    return ((k == null) ? 0 : k.hashCode()) ^ ((v == null) ? 0 : v.hashCode());
  }

  /**
   * A sensible definition of {@link #toString} in terms of {@link
   * #getKey} and {@link #getValue}. If you override either of these
   * methods, you may wish to override {@link #equals} to forward to this
   * implementation.
   *
   * @since 7.0
   */
  @Beta protected String standardToString() {
    return getKey() + "=" + getValue();
  }
}