OCP 17 Study Guide Chapter 7 Review Question 20

[Marco Olivi]

Hello,
The question asks to determine if that statement is true or false:

Given an arbitrary instance of a class, it cannot be determined until runtime which overridden method will be executed in a parent class.

and the answer is true:

Part of polymorphism is that methods are replaced through overriding wherever they are called, regardless of whether they're in a parent or child class. For this reason, option A is correct,

I might don't understand the question and the meaning of "arbitrary instance".

It's very clear to me how overriding works, and I would say that all of overridden methods called inside the parent class will be executed, despite the reference type of the instance. So we already know before runtime.

[Ankit Garg]

Howdy Marco, let's take a look at it with an example. Let's say you have this parent class:

class Parent {
  void print() {
    System.out.println("Message: " + getMessage());
  }
  String getMessage() {
    return "Parent class";
  }
}
class Child extends Parent {
  String getMessage() {
    return "Child class";
  }
}
class Caller {
  void call(Parent target) {
    target.print();
  }
}

Now looking only at the above code, can you say for sure whenever print method is called it will always print Message: Parent class? I might call the call method with new Parent() as parameter OR I might send a new Child() to the call method. Or I might create another class which extends Parent. I'm pretty sure that's what the question meant to say. When you look only at your class (Parent in this case), you don't know who will inherit from it (someone else 6 months later might inherit from your class), so you cannot be sure which method call will be overridden.

[Marco Olivi]

Thank you very much Ankit!
Now I understand what the question was asking for, and the meaning of "arbitrary instance".
Thanks!