Question

My question is regarding Inheritance and Polymorphism.

I have a class A which is the super class. Classes B and C extends A and are subclasses.

Now i have a function in other class(not A or B or C) that takes argument of type A. So polymorphism allows us to pass the argument as A , B or C to that function. Now in that function, inorder to access B‘s properties, i have to typecast the received argument to B.

Is type casting a good programming technique ? If not what is the use of polymorphism here ?

Thanks.

Answer 1

Typecasting is only useful if you know what to cast into.

The use of polymorphism here is that the method in the other class can be written in a generic way, and a single piece of code can thus operate on different types, which, though, have some interface (class A) in common.

So… typecasting is (depending on language culture) a very bad practice. Use of polymorphism is preferred (depending…), such as creating a different method with signature ->(B), taking an argument of type B.

Update: added Java pseudocode

class A { int m() {} }
class B extends A { int b() {} }
class C extends A {}
class $ { 
  static void x(A $$) {} 
  static void x(B $$) {}
}
public class P {
  public static void main(String[] a) {
    $.x(new A()); //calls $::x(A)
    $.x(new B()); //calls $::x(B)
  }
}