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

Arrays.stream(int[]) creates an IntStream, not a Stream<Integer>. 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(", "));