Java Runtime Polymorphism

In runtime polymorphism ... the code is called at run time according to need or given conditions.
suppose there r two methods namely Add() one in super class and other is in sub class.both have the same name and same parameters.
so we have to choose that which method from them shld called at run time i.e. of super class or of sub class.by polymorphism we do that.
ex:-
 class A
{
int add(){//code of the method}
//some other code
}
class B extends A
{
int add(){//code of the method}
//some other code
}
class AB
{
public static void main(String s[])
{
A ob1;
ob1=new A();
int i=ob1.add();//will call the method of super class.
ob1=new B();// sub class's reference can be assigned to super class address but not vice versa.to do that we have to type cast the reference of the sub class in reference of the super class.
int j=ob1.add();//will call the method of sub class
}
}