public class Test {

  public static void main(String[] args) {

   Base b; // declare b
   if (Math.random()<0.5) // maybe..
     b=new Sub1(); // b is an instance of Sub1..
   else
     b=new Sub2(); // or Sub2

   b.method(); // which method?

  }
}

class Base
{
  void method() {System.out.println("Base class");}
}

class Sub1 extends Base
{
  void method() {System.out.println("Sub1 class");}
}

class Sub2 extends Base
{
  void method() {System.out.println("Sub2 class");}
}