How to fix ambiguous type on method reference (toString of an Integer)?

Question

When doing this

Stream.of(1, 32, 12, 15, 23).map(Integer::toString);

I get an ambiguous type error. Understandably, the compiler can't tell if I mean toString(int) or toString() from Integer.

When not using a method reference, I might have gotten out of this with an explicit cast or write out the generics long hand, but how can I let the compiler know what I mean here? What syntax (if any) can I use to make in unambiguous?

Answer 1

There is no way to make method references unambiguous; simply said, method references are a feature that is just supported for unambiguous method references only. So you have two solutions:

1. use a lambda expression:

   Stream.of(1, 32, 12, 15, 23).map(i->Integer.toString(i));
   

2. (preferred, at least by me) Use a stream of primitive int values when the source consists of primitive int values only:

   IntStream.of(1, 32, 12, 15, 23).mapToObj(Integer::toString);
   

   This will use the static Integer.toString(int) method for consuming the int values.

Answer 2

Since Integer.toString() overrides Object.toString(), in this particular case you could fix it as follows:

Stream.of(1, 32, 12, 15, 23).map(Object::toString);

Answer 3

Your main options, using method references, are:

Stream.of(1, 32, 12, 15, 23).map(String::valueOf);
IntStream.of(1, 32, 12, 15, 23).mapToObj(Integer::toString);

Your current version could mean i -> i.toString() or i -> Integer.toString(i).