IntFunction<String> and Function<Integer, String>

Question

I have following two simple code:

IntFunction<String> f1 = Integer::toString;
Function<Integer, String> f2 = Integer::toString;

I thought that both definitions are correct and equivalently the same thing, but the second one has compiling errors, complaining that Required Function<Integer, String>,but Method Reference is found.

Answer 1

The second method reference is ambiguous:

both the static method

public static String toString(int i)

and the instance method

public String toString()

are applicable.

If you write the second assignment using lambda expressions, you can see there are two methods you can use:

Function<Integer, String> f2 = i -> Integer.toString (i);

or

Function<Integer, String> f2 = i -> i.toString ();

when you assign Integer::toString, the compiler can't decide which method you are referring to.

On the other hand, in the case of IntFunction<String>, only public static String toString(int i) is applicable.