Question

Class A calls the public method f() in the Constructor. Class B overrides method f() with its own implementation.

Suppose you intantiate Object B.. method f() of Object B would be called in the Constructor of Object A, although Object B is not fully initialized.

Can anyone explain this behavior?

EDIT: Yes, it’s not recommended practice.. yet i don’t understand why Java is not calling the f() implementation of the base-Class A instead of “reaching out” to the f() implementation of the derived Class B.

Code:

class A {
    A() {
        System.out.println("A: constructor");
        f();
    }

    public void f() {
        System.out.println("A: f()");
    }
}

class B extends A {
    int x = 10;
    B() {
        System.out.println("B: constructor");
    }

    @Override
    public void f() {
        System.out.println("B: f()");
        this.x++;
        System.out.println("B: x = " + x);

    }
}

public class PolyMethodConst {
    public static void main(String[] args) {
        new B();
    }
}

Output:

A: constructor
B: f()
B: x = 1
B: constructor

Answer 1

You’re correct, that is the way it works. It’s not recommended practice though, because somebody who inherits from your class can unintentionally break it.