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.Comparator;
import java.util.Map;
import java.util.Set;
import java.util.SortedSet;
import org.checkerframework.checker.nullness.qual.Nullable;


A SetMultimap whose set of values for a given key are kept sorted; that is, they comprise a SortedSet. It cannot hold duplicate key-value pairs; adding a key-value pair that's already in the multimap has no effect. This interface does not specify the ordering of the multimap's keys. See the Multimap documentation for information common to all multimaps. 
The get, removeAll, and replaceValues methods each return a SortedSet of values, while Multimap.entries() returns a Set of map entries. Though the method signature doesn't say so explicitly, the map returned by asMap has SortedSet values. 
See the Guava User Guide article on  
Multimap.

Author:Jared Levy
Since:2.0
/**
 * A {@code SetMultimap} whose set of values for a given key are kept sorted; that is, they comprise
 * a {@link SortedSet}. It cannot hold duplicate key-value pairs; adding a key-value pair that's
 * already in the multimap has no effect. This interface does not specify the ordering of the
 * multimap's keys. 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
 * SortedSet} of values, while {@link Multimap#entries()} returns a {@link Set} of map entries.
 * Though the method signature doesn't say so explicitly, the map returned by {@link #asMap} has
 * {@code SortedSet} values.
 *
 * <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 SortedSetMultimap<K, V> extends SetMultimap<K, V> {
  // Following Javadoc copied from Multimap.

  
Returns a collection view of all values associated with a key. If no mappings in the multimap
have the provided key, an empty collection is returned.

Changes to the returned collection will update the underlying multimap, and vice versa.

Because a SortedSetMultimap has unique sorted values for a given key, this method returns a SortedSet, instead of the Collection specified in the Multimap interface. 
/**
   * Returns a collection view of all values associated with a key. If no mappings in the multimap
   * have the provided key, an empty collection is returned.
   *
   * <p>Changes to the returned collection will update the underlying multimap, and vice versa.
   *
   * <p>Because a {@code SortedSetMultimap} has unique sorted values for a given key, this method
   * returns a {@link SortedSet}, instead of the {@link java.util.Collection} specified in the
   * {@link Multimap} interface.
   */
  @Override
  SortedSet<V> get(@Nullable K key);

  
Removes all values associated with a given key.

Because a SortedSetMultimap has unique sorted values for a given key, this method returns a SortedSet, instead of the Collection specified in the Multimap interface. 
/**
   * Removes all values associated with a given key.
   *
   * <p>Because a {@code SortedSetMultimap} has unique sorted values for a given key, this method
   * returns a {@link SortedSet}, instead of the {@link java.util.Collection} specified in the
   * {@link Multimap} interface.
   */
  @CanIgnoreReturnValue
  @Override
  SortedSet<V> removeAll(@Nullable Object key);

  
Stores a collection of values with the same key, replacing any existing values for that key.

Because a SortedSetMultimap has unique sorted values for a given key, this method returns a SortedSet, instead of the Collection specified in the Multimap interface. 
Any duplicates in values will be stored in the multimap once. 
/**
   * Stores a collection of values with the same key, replacing any existing values for that key.
   *
   * <p>Because a {@code SortedSetMultimap} has unique sorted values for a given key, this method
   * returns a {@link SortedSet}, 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
  SortedSet<V> replaceValues(K key, Iterable<? extends V> values);

  
Returns a map view that associates each key with the corresponding values in the multimap. Changes to the returned map, such as element removal, will update the underlying multimap. The map does not support setValue() on its entries, put, or putAll. 
When passed a key that is present in the map, asMap().get(Object) has the same behavior as get, returning a live collection. When passed a key that is not present, however, asMap().get(Object) returns null instead of an empty collection. 
Note: The returned map's values are guaranteed to be of type SortedSet. To obtain this map with the more specific generic type Map<K, SortedSet<V>>, call Multimaps.asMap(SortedSetMultimap<Object,Object>) instead. 
/**
   * Returns a map view that associates each key with the corresponding values in the multimap.
   * Changes to the returned map, such as element removal, will update the underlying multimap. The
   * map does not support {@code setValue()} on its entries, {@code put}, or {@code putAll}.
   *
   * <p>When passed a key that is present in the map, {@code asMap().get(Object)} has the same
   * behavior as {@link #get}, returning a live collection. When passed a key that is not present,
   * however, {@code asMap().get(Object)} returns {@code null} instead of an empty collection.
   *
   * <p><b>Note:</b> The returned map's values are guaranteed to be of type {@link SortedSet}. To
   * obtain this map with the more specific generic type {@code Map<K, SortedSet<V>>}, call {@link
   * Multimaps#asMap(SortedSetMultimap)} instead.
   */
  @Override
  Map<K, Collection<V>> asMap();

  
Returns the comparator that orders the multimap values, with null indicating that natural ordering is used. 
/**
   * Returns the comparator that orders the multimap values, with {@code null} indicating that
   * natural ordering is used.
   */
  Comparator<? super V> valueComparator();
}