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

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.