Why do two methods with signature (primitive, wrapper) and (primitive, primitive) cause the method call (wrapper, primitive) to be ambiguous?

Question

It's just an exercise but I can't figure out the ambiguity:

private static void flipFlop(String str, int i, Integer iRef) {
System.out.println(str + "ciao");
}

private static void flipFlop(String str, int i, int j) {
System.out.println(str + "hello");
}

public static void main(String[] args) {
flipFlop("hello", new Integer(4), 2004);
}

It says:

The method flipFlop(String, int, Integer) is ambiguous for the type Test

I would have guessed that the second argument would have been unwrapped to int and so the second flipFlop method would have been the choice.

Answer 1

If you enjoy riveting reading, here is the relevant portion of the Java Language specification that describes how methods are resolved.

But basically your third argument can be interpreted as a primitive or an autoboxed wrapper, and the compiler can't figure out what you want. Both methods are "maximally specific" to use the JLS terminology.