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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more.

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/*
 * 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.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import javax.annotation.Nullable;

/**
 * A {@code Multimap} that can hold duplicate key-value pairs and that maintains
 * the insertion ordering of values for a given key. See the {@link Multimap}
 * documentation for information common to all multimaps.
 *
 * The {@link #get}, {@link #removeAll}, and {@link #replaceValues} methods
 * each return a {@link List} of values. Though the method signature doesn't say
 * so explicitly, the map returned by {@link #asMap} has {@code List} values.
 *
 * 
See the Guava User Guide article on 
 * {@code Multimap}.
 *
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible
public interface ListMultimap extends Multimap {
  /**
   * {@inheritDoc}
   *
   * 
Because the values for a given key may have duplicates and follow the
   * insertion ordering, this method returns a {@link List}, instead of the
   * {@link java.util.Collection} specified in the {@link Multimap} interface.
   */
  @Override
  List get(@Nullable K key);

  /**
   * {@inheritDoc}
   *
   * 
Because the values for a given key may have duplicates and follow the
   * insertion ordering, this method returns a {@link List}, instead of the
   * {@link java.util.Collection} specified in the {@link Multimap} interface.
   */
  @CanIgnoreReturnValue
  @Override
  List removeAll(@Nullable Object key);

  /**
   * {@inheritDoc}
   *
   * 
Because the values for a given key may have duplicates and follow the
   * insertion ordering, this method returns a {@link List}, instead of the
   * {@link java.util.Collection} specified in the {@link Multimap} interface.
   */
  @CanIgnoreReturnValue
  @Override
  List replaceValues(K key, Iterable values);

  /**
   * {@inheritDoc}
   *
   * 
Note: The returned map's values are guaranteed to be of type
   * {@link List}. To obtain this map with the more specific generic type
   * {@code Map>}, call {@link Multimaps#asMap(ListMultimap)}
   * instead.
   */
  @Override
  Map> asMap();

  /**
   * Compares the specified object to this multimap for equality.
   *
   * 
Two {@code ListMultimap} instances are equal if, for each key, they
   * contain the same values in the same order. If the value orderings disagree,
   * the multimaps will not be considered equal.
   *
   * An empty {@code ListMultimap} is equal to any other empty {@code
   * Multimap}, including an empty {@code SetMultimap}.
   */
  @Override
  boolean equals(@Nullable Object obj);
}