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

Question

This code works (taken in the Javadoc):

List<Integer> numbers = Arrays.asList(1, 2, 3, 4); String commaSeparatedNumbers = numbers.stream()     .map(i -> i.toString())     .collect(Collectors.joining(", ")); 

This one can't be compiled:

int[] numbers = {1, 2, 3, 4}; String commaSeparatedNumbers = Arrays.stream(numbers)     .map((Integer i) -> i.toString())     .collect(Collectors.joining(", ")); 

IDEA tells me I have an "incompatible return type String in lambda expression".

Why ? And how to fix that ?

Answer 1

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(", "));