guava/src/com/google/common/collect/Multimaps.java
public static <K, V> SortedSetMultimap<K, V> synchronizedSortedSetMultimap(
      SortedSetMultimap<K, V> multimap) {
    return Synchronized.sortedSetMultimap(multimap, null);
  }
public static <K, V> SortedSetMultimap<K, V> synchronizedSortedSetMultimap(
      SortedSetMultimap<K, V> multimap) {
    return Synchronized.sortedSetMultimap(multimap, null);
  }
Returns a synchronized (thread-safe) {@code SortedSetMultimap} backed by the specified
multimap.

<p>You must follow the warnings described in {@link #synchronizedMultimap}.

<p>The returned multimap will be serializable if the specified multimap is serializable.

@param multimap the multimap to be wrapped
@return a synchronized view of the specified multimap