Why can't I map integers to strings when streaming from an array?

Question

This code works (taken in the Javadoc):

List numbers = Arrays.asList(1, 2, 3, 4);
String commaSeparatedNumbers = numbers.stream()
    .map(i -         


        

Answer 1

Arrays.stream(int[]) creates an IntStream, not a Stream. So you need to call mapToObj instead of just map, when mapping an int to an object.

This should work as expected:

String commaSeparatedNumbers = Arrays.stream(numbers)
    .mapToObj(i -> ((Integer) i).toString()) //i is an int, not an Integer
    .collect(Collectors.joining(", "));

which you can also write:

String commaSeparatedNumbers = Arrays.stream(numbers)
    .mapToObj(Integer::toString)
    .collect(Collectors.joining(", "));