Question

In two days i have an exam in java, and i can not figure out the answer to this question:

class ClassA {
 public String foo(Integer x , int y) {
  return "Integer, int";
 }
 public String foo(int x, Double y) {
  return "int, Double";
 }
 public String foo(Number x, Number y) {
  return "Number, Number";
 }
 public String foo(Object x, Object y) {
  return "Object, Object";
 }
 public static void main(String... args) {
  ClassA a = new ClassA();
  System.out.print(a.foo(5, 1.2f) + " ");
  System.out.println(a.foo(null, null));
 }
}

What’s the output?

The Answer is:

Number, Number Number, Number

I know that java always chooses the most specified Method, that is why a.foo(null,null); will envoke the Number,Number Method and not the Object,Object Method.
But why does a.foo(5,1.2f); also envoke the Number,Number Method and not the int,Double Method??

But one more thing which might be helpful:
If i remove the f after 1.2, so that the call is:
a.foo(5,1.2);
I get a compiler error, that it can not choose between the Number,Number and int,Double Method…

Would be really helpful, if you guys could explain that to me 🙂

Answer 1

1.2f is not wrapped by a Double, it’s wrapped by a Float. SinceFloat is not a subclass of Double (they are distinct subclasses of Number), the most specific method signature that can be used is foo(Number,Number).

Once you remove the f, 1.2 will be treated a double (the primitive, not the wrapper class) by default, which can be autoboxed to a Double. However the 5 can also be autoboxed to an Integer, thus causing the ambiguity.