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Guava Collections is a collections extension library with expanded collection types, implementations, and utility methods for dealing with aggregate data.

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/*
 * Copyright (C) 2007 Google Inc.
 *
 * 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 java.util.Collection;
import java.util.Map;
import java.util.Set;

import javax.annotation.Nullable;

/**
 * A {@code Multimap} that cannot hold duplicate key-value pairs. Adding a
 * key-value pair that's already in the multimap has no effect.
 *
 * The {@link #get}, {@link #removeAll}, and {@link #replaceValues} methods
 * each return a {@link Set} of values, while {@link #entries} returns a {@code
 * Set} of map entries. Though the method signature doesn't say so explicitly,
 * the map returned by {@link #asMap} has {@code Set} values.
 *
 * @author Jared Levy
 * @since 2 (imported from Google Collections Library)
 */
@GwtCompatible
public interface SetMultimap extends Multimap {
  /**
   * {@inheritDoc}
   *
   * 
Because a {@code SetMultimap} has unique values for a given key, this
   * method returns a {@link Set}, instead of the {@link java.util.Collection}
   * specified in the {@link Multimap} interface.
   */
  Set get(@Nullable K key);

  /**
   * {@inheritDoc}
   *
   * 
Because a {@code SetMultimap} has unique values for a given key, this
   * method returns a {@link Set}, instead of the {@link java.util.Collection}
   * specified in the {@link Multimap} interface.
   */
  Set removeAll(@Nullable Object key);

  /**
   * {@inheritDoc}
   *
   * 
Because a {@code SetMultimap} has unique values for a given key, this
   * method returns a {@link Set}, instead of the {@link java.util.Collection}
   * specified in the {@link Multimap} interface.
   *
   * 
Any duplicates in {@code values} will be stored in the multimap once.
   */
  Set replaceValues(K key, Iterable values);

  /**
   * {@inheritDoc}
   *
   * 
Because a {@code SetMultimap} has unique values for a given key, this
   * method returns a {@link Set}, instead of the {@link java.util.Collection}
   * specified in the {@link Multimap} interface.
   */
  Set> entries();

  /**
   * {@inheritDoc}
   *
   * 
Though the method signature doesn't say so explicitly, the returned map
   * has {@link Set} values.
   */
  Map> asMap();

  /**
   * Compares the specified object to this multimap for equality.
   *
   * Two {@code SetMultimap} instances are equal if, for each key, they
   * contain the same values. Equality does not depend on the ordering of keys
   * or values.
   */
  boolean equals(@Nullable Object obj);
}