java有关继承的问题

Question

public class SuperClass {
public int add(int i,int j)
{
return i+j;
}
public double add(double i,double j)
{
return i+j;
}
 
}
 
 
public class SubClass extends SuperClass {
public int add(int i,int j)
{
return 2*(i+j);
}
public double add(double i,double j)
{
return 2*(i+j);
}
 
public String add(String i,String j)
{
return i+j;
}
}
 
 
public class Test4 {
 
public static void main(String[] args) {
SubClass objs=new SubClass();
System.out.println(objs.add('a','b')); 
 
}
 
}

Answer 1

问题是？