Why is this usage of Stream::flatMap wrong?

Question

I expected to be able to use Stream::flatMap like this

public static List duplicate(String s) {

    List l = new ArrayList

        

Answer 1

The lambda expression in flatMap needs to return a Stream, as can be seen by the argument of flatMap which is of type Function<? super T, ? extends Stream<? extends R>>.

The following code will compile and run fine:

listOfStrings.stream()
             .flatMap(str -> duplicate(str).stream()) // note the .stream() here
             .collect(Collectors.toList());

because the lambda expression str -> duplicate(str).stream() is of type Function<String, Stream<String>>.

Answer 2

If you want to duplicate each object in the stream several times, you don't need to waste memory on this with additional ArrayList. There are several shorter and faster alternatives.

• Generate new stream using Stream.generate, then limit it:

  listOfStrings.stream()
               .flatMap(str -> Stream.generate(() -> str).limit(2))
               .collect(Collectors.toList());
  

• Generate sequence of numbers via IntStream.range and map them to the same string:

  listOfStrings.stream()
               .flatMap(str -> IntStream.range(0, 2).mapToObj(i -> str))
               .collect(Collectors.toList());
  

• Use good old Collections.nCopies:

  listOfStrings.stream()
               .flatMap(str -> Collections.nCopies(2, str).stream())
               .collect(Collectors.toList());
  

If you are sure that you will always duplicate exactly two times, there's the shortest alternative:

listOfStrings.stream()
             .flatMap(str -> Stream.of(str, str))
             .collect(Collectors.toList());