import java.util.function.Function; import java.util.function.ToIntFunction; import java.util.function.ToLongFunction; import java.util.stream.IntStream; import java.util.stream.LongStream; interface Test { default IntStream distinct2(Stream s) { return s.map(i -> i.length()); } } interface Stream { Stream map(Function mapper); IntStream map(ToIntFunction mapper); LongStream map(ToLongFunction mapper); }