Why method overloading is not possible by changing the return type in java?

n java, method overloading is not possible by changing the return type of the method because there may occur ambiguity.here is an example which state how ambiguity might occur:

here how java determine which method to call

class Calculation3{  
  int sum(int a,int b){System.out.println(a+b);}  
  double sum(int a,int b){System.out.println(a+b);}  
  
  public static void main(String args[]){  
  Calculation3 obj=new Calculation3();  
  int result=obj.sum(20,20); //Compile Time Error  
  
  }  
}