is Method overloading possible by changing the return type of method ? Why ?

No, as it leads to ambiguity :-S

Example

class Test { 
  int sum(int a, int b) {

     int x = a+b;

  } 
  

  double sum(int a, int b){

     double x = a+b;

  } 

  public static void main(String args[]){ 
    Test test = new Test(); 
    int result = test.sum(20,20);   // Compile time Error

               // Ambiguity : which method to call ?

               // As both have same type and sequence of argument 
  } 
}