import java.util.List; import java.util.stream.Stream; import java.util.stream.Collectors; import java.util.TreeSet; public class Test{ public static void main(String...s){ List<String> collected = Stream.of("a","b","c","c").collect(Collectors.toList()); TreeSet<String> treeSet = collected.stream().collect(Collectors.toCollection(TreeSet::new)); System.out.println(collected); System.out.println(treeSet); } }