Denys Séguret Denys Séguret - 1 month ago 7
Java Question

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

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

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(", "));