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Java example source code file (ForwardingMap.java)

This example Java source code file (ForwardingMap.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

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Java - Java tags/keywords

annotation, beta, canignorereturnvalue, forwardingmap, gwtcompatible, iterator, map, object, override, standardentryset, standardkeyset, standardvalues, string, util

The ForwardingMap.java Java example source code

/*
 * 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 com.google.errorprone.annotations.CanIgnoreReturnValue;

import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

import javax.annotation.Nullable;

/**
 * A map which forwards all its method calls to another map. Subclasses should
 * override one or more methods to modify the behavior of the backing map as
 * desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p>Warning: The methods of {@code ForwardingMap} forward
 * <i>indiscriminately to the methods of the delegate. For example,
 * overriding {@link #put} alone <i>will not change the behavior of {@link
 * #putAll}, which can lead to unexpected behavior. In this case, you should
 * override {@code putAll} as well, either providing your own implementation, or
 * delegating to the provided {@code standardPutAll} method.
 *
 * <p>{@code default} method warning: This class does not forward calls to {@code
 * default} methods. Instead, it inherits their default implementations. When those implementations
 * invoke methods, they invoke methods on the {@code ForwardingMap}.
 *
 * <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 a {@code SortedMap} whose comparator is not consistent
 * with {@code equals}.
 *
 * <p>The {@code standard} methods and the collection views they return are not
 * guaranteed to be thread-safe, even when all of the methods that they depend
 * on are thread-safe.
 *
 * @author Kevin Bourrillion
 * @author Jared Levy
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
public abstract class ForwardingMap<K, V> extends ForwardingObject implements Map {
  // TODO(lowasser): identify places where thread safety is actually lost

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

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

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

  @Override
  public boolean isEmpty() {
    return delegate().isEmpty();
  }

  @CanIgnoreReturnValue
  @Override
  public V remove(Object object) {
    return delegate().remove(object);
  }

  @Override
  public void clear() {
    delegate().clear();
  }

  @Override
  public boolean containsKey(@Nullable Object key) {
    return delegate().containsKey(key);
  }

  @Override
  public boolean containsValue(@Nullable Object value) {
    return delegate().containsValue(value);
  }

  @Override
  public V get(@Nullable Object key) {
    return delegate().get(key);
  }

  @CanIgnoreReturnValue
  @Override
  public V put(K key, V value) {
    return delegate().put(key, value);
  }

  @Override
  public void putAll(Map<? extends K, ? extends V> map) {
    delegate().putAll(map);
  }

  @Override
  public Set<K> keySet() {
    return delegate().keySet();
  }

  @Override
  public Collection<V> values() {
    return delegate().values();
  }

  @Override
  public Set<Entry entrySet() {
    return delegate().entrySet();
  }

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

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

  /**
   * A sensible definition of {@link #putAll(Map)} in terms of {@link
   * #put(Object, Object)}. If you override {@link #put(Object, Object)}, you
   * may wish to override {@link #putAll(Map)} to forward to this
   * implementation.
   *
   * @since 7.0
   */
  protected void standardPutAll(Map<? extends K, ? extends V> map) {
    Maps.putAllImpl(this, map);
  }

  /**
   * A sensible, albeit inefficient, definition of {@link #remove} in terms of
   * the {@code iterator} method of {@link #entrySet}. If you override {@link
   * #entrySet}, you may wish to override {@link #remove} to forward to this
   * implementation.
   *
   * <p>Alternately, you may wish to override {@link #remove} with {@code
   * keySet().remove}, assuming that approach would not lead to an infinite
   * loop.
   *
   * @since 7.0
   */
  @Beta
  protected V standardRemove(@Nullable Object key) {
    Iterator<Entry entryIterator = entrySet().iterator();
    while (entryIterator.hasNext()) {
      Entry<K, V> entry = entryIterator.next();
      if (Objects.equal(entry.getKey(), key)) {
        V value = entry.getValue();
        entryIterator.remove();
        return value;
      }
    }
    return null;
  }

  /**
   * A sensible definition of {@link #clear} in terms of the {@code iterator}
   * method of {@link #entrySet}. In many cases, you may wish to override
   * {@link #clear} to forward to this implementation.
   *
   * @since 7.0
   */
  protected void standardClear() {
    Iterators.clear(entrySet().iterator());
  }

  /**
   * A sensible implementation of {@link Map#keySet} in terms of the following
   * methods: {@link ForwardingMap#clear}, {@link ForwardingMap#containsKey},
   * {@link ForwardingMap#isEmpty}, {@link ForwardingMap#remove}, {@link
   * ForwardingMap#size}, and the {@link Set#iterator} method of {@link
   * ForwardingMap#entrySet}. In many cases, you may wish to override {@link
   * ForwardingMap#keySet} to forward to this implementation or a subclass
   * thereof.
   *
   * @since 10.0
   */
  @Beta
  protected class StandardKeySet extends Maps.KeySet<K, V> {
    /** Constructor for use by subclasses. */
    public StandardKeySet() {
      super(ForwardingMap.this);
    }
  }

  /**
   * A sensible, albeit inefficient, definition of {@link #containsKey} in terms
   * of the {@code iterator} method of {@link #entrySet}. If you override {@link
   * #entrySet}, you may wish to override {@link #containsKey} to forward to
   * this implementation.
   *
   * @since 7.0
   */
  @Beta
  protected boolean standardContainsKey(@Nullable Object key) {
    return Maps.containsKeyImpl(this, key);
  }

  /**
   * A sensible implementation of {@link Map#values} in terms of the following
   * methods: {@link ForwardingMap#clear}, {@link ForwardingMap#containsValue},
   * {@link ForwardingMap#isEmpty}, {@link ForwardingMap#size}, and the {@link
   * Set#iterator} method of {@link ForwardingMap#entrySet}. In many cases, you
   * may wish to override {@link ForwardingMap#values} to forward to this
   * implementation or a subclass thereof.
   *
   * @since 10.0
   */
  @Beta
  protected class StandardValues extends Maps.Values<K, V> {
    /** Constructor for use by subclasses. */
    public StandardValues() {
      super(ForwardingMap.this);
    }
  }

  /**
   * A sensible definition of {@link #containsValue} in terms of the {@code
   * iterator} method of {@link #entrySet}. If you override {@link #entrySet},
   * you may wish to override {@link #containsValue} to forward to this
   * implementation.
   *
   * @since 7.0
   */
  protected boolean standardContainsValue(@Nullable Object value) {
    return Maps.containsValueImpl(this, value);
  }

  /**
   * A sensible implementation of {@link Map#entrySet} in terms of the following
   * methods: {@link ForwardingMap#clear}, {@link ForwardingMap#containsKey},
   * {@link ForwardingMap#get}, {@link ForwardingMap#isEmpty}, {@link
   * ForwardingMap#remove}, and {@link ForwardingMap#size}. In many cases, you
   * may wish to override {@link #entrySet} to forward to this implementation
   * or a subclass thereof.
   *
   * @since 10.0
   */
  @Beta
  protected abstract class StandardEntrySet extends Maps.EntrySet<K, V> {
    /** Constructor for use by subclasses. */
    public StandardEntrySet() {}

    @Override
    Map<K, V> map() {
      return ForwardingMap.this;
    }
  }

  /**
   * A sensible definition of {@link #isEmpty} in terms of the {@code iterator}
   * method of {@link #entrySet}. If you override {@link #entrySet}, you may
   * wish to override {@link #isEmpty} to forward to this implementation.
   *
   * @since 7.0
   */
  protected boolean standardIsEmpty() {
    return !entrySet().iterator().hasNext();
  }

  /**
   * A sensible definition of {@link #equals} in terms of the {@code equals}
   * method of {@link #entrySet}. If you override {@link #entrySet}, you may
   * wish to override {@link #equals} to forward to this implementation.
   *
   * @since 7.0
   */
  protected boolean standardEquals(@Nullable Object object) {
    return Maps.equalsImpl(this, object);
  }

  /**
   * A sensible definition of {@link #hashCode} in terms of the {@code iterator}
   * method of {@link #entrySet}. If you override {@link #entrySet}, you may
   * wish to override {@link #hashCode} to forward to this implementation.
   *
   * @since 7.0
   */
  protected int standardHashCode() {
    return Sets.hashCodeImpl(entrySet());
  }

  /**
   * A sensible definition of {@link #toString} in terms of the {@code iterator}
   * method of {@link #entrySet}. If you override {@link #entrySet}, you may
   * wish to override {@link #toString} to forward to this implementation.
   *
   * @since 7.0
   */
  protected String standardToString() {
    return Maps.toStringImpl(this);
  }
}

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