https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * 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.Map;
import java.util.Map.Entry;
import java.util.Set;
import org.checkerframework.checker.nullness.qual.Nullable;


A Multimap that cannot hold duplicate key-value pairs. Adding a key-value pair that's already in the multimap has no effect. See the Multimap documentation for information common to all multimaps. 
The get, removeAll, and replaceValues methods each return a Set of values, while entries returns a Set of map entries. Though the method signature doesn't say so explicitly, the map returned by asMap has Set values. 
If the values corresponding to a single key should be ordered according to a Comparator (or the natural order), see the SortedSetMultimap subinterface. 
Since the value collections are sets, the behavior of a SetMultimap is not specified if key or value objects already present in the multimap change in a manner that affects equals comparisons. Use caution if mutable objects are used as keys or values in a 
SetMultimap. 
See the Guava User Guide article on  
Multimap.

Author:Jared Levy
Since:2.0
/**
 * A {@code Multimap} that cannot hold duplicate key-value pairs. Adding a key-value pair that's
 * already in the multimap has no effect. See the {@link Multimap} documentation for information
 * common to all multimaps.
 *
 * <p>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.
 *
 * <p>If the values corresponding to a single key should be ordered according to a {@link
 * java.util.Comparator} (or the natural order), see the {@link SortedSetMultimap} subinterface.
 *
 * <p>Since the value collections are sets, the behavior of a {@code SetMultimap} is not specified
 * if key <em>or value</em> objects already present in the multimap change in a manner that affects
 * {@code equals} comparisons. Use caution if mutable objects are used as keys or values in a {@code
 * SetMultimap}.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multimap"> {@code
 * Multimap}</a>.
 *
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible
public interface SetMultimap<K, V> extends Multimap<K, V> {
  
{@inheritDoc}

Because a SetMultimap has unique values for a given key, this method returns a Set, instead of the Collection specified in the Multimap interface. 
/**
   * {@inheritDoc}
   *
   * <p>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.
   */
  @Override
  Set<V> get(@Nullable K key);

  
{@inheritDoc}

Because a SetMultimap has unique values for a given key, this method returns a Set, instead of the Collection specified in the Multimap interface. 
/**
   * {@inheritDoc}
   *
   * <p>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.
   */
  @CanIgnoreReturnValue
  @Override
  Set<V> removeAll(@Nullable Object key);

  
{@inheritDoc}

Because a SetMultimap has unique values for a given key, this method returns a Set, instead of the Collection specified in the Multimap interface. 
Any duplicates in values will be stored in the multimap once. 
/**
   * {@inheritDoc}
   *
   * <p>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.
   *
   * <p>Any duplicates in {@code values} will be stored in the multimap once.
   */
  @CanIgnoreReturnValue
  @Override
  Set<V> replaceValues(K key, Iterable<? extends V> values);

  
{@inheritDoc}

Because a SetMultimap has unique values for a given key, this method returns a Set, instead of the Collection specified in the Multimap interface. 
/**
   * {@inheritDoc}
   *
   * <p>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.
   */
  @Override
  Set<Entry<K, V>> entries();

  
{@inheritDoc}

Note: The returned map's values are guaranteed to be of type Set. To obtain this map with the more specific generic type Map<K, Set<V>>, call Multimaps.asMap(SetMultimap<Object,Object>) instead. 
/**
   * {@inheritDoc}
   *
   * <p><b>Note:</b> The returned map's values are guaranteed to be of type {@link Set}. To obtain
   * this map with the more specific generic type {@code Map<K, Set<V>>}, call {@link
   * Multimaps#asMap(SetMultimap)} instead.
   */
  @Override
  Map<K, Collection<V>> asMap();

  
Compares the specified object to this multimap for equality.

Two SetMultimap instances are equal if, for each key, they contain the same values. Equality does not depend on the ordering of keys or values. 
An empty SetMultimap is equal to any other empty Multimap, including an empty ListMultimap. 
/**
   * Compares the specified object to this multimap for equality.
   *
   * <p>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.
   *
   * <p>An empty {@code SetMultimap} is equal to any other empty {@code Multimap}, including an
   * empty {@code ListMultimap}.
   */
  @Override
  boolean equals(@Nullable Object obj);
}