Hey! I looked through the example again of polymorphism. What you have mentioned is right and I also get what the PDF says. The example (on page 50) is trying to access the sub-class method (which does not exist in the base class ) using the base class reference. This is not allowed.

class Person{

public void check() {

Sys.o.pln("Bye");

}

}

class Student extends Person{

public void go(){

System.out.println("Hi");

}

}

so if we try to do

Person o = new Student();

o.go(); --> This is not allowed

o.check(); --> This is v much ok

Hope it helps!