import java.util.List;
import java.util.Set;
import java.util.concurrent.ConcurrentMap;
import java.util.stream.Collectors;
import java.util.stream.Stream;

class Test {
    public static void main(String[] args) {
        Stream<String> ohMy = Stream.of("lions", "tigers", "bears").parallel();
        ConcurrentMap<Integer, List<String>> map = ohMy.collect(
                Collectors.groupingByConcurrent(String::length));
        System.out.println(map); // {5=[lions, bears], 6=[tigers]}
    }
}