Which constructor initializes the variable x3?

Question

I have found similar question but I am not sure that answer is correct. I had that question on my java exam.

Which constructor initializes the variable x3?

class X {
    int x1, x2, x3;
}

class Y extends X {
    int y1;
    Y() {
        x1 = 1;
        x2 = 2;
        y1 = 10;
    }
}

class Z extends Y {
    int z1;
    Z() {
        x1 = 3;
        y1 = 20;
        z1 = 100;
    }
}

public class Test3 {
    public static void main(String[] args) {
        Z obj = new Z();
        System.out.println(obj.x3 + ", " + obj.y1 + ", " +obj.z1);
    }
}

Answers:

• A. Only the default constructor of class X
• B. Only the no-argument constructor of class Y
• C. Only the no-argument constructor of class Z
• D. Only the default constructor of object class

Thank you for help.

Answer 1

A subclass inherits all the fields of its superclass. So when the constructor of the subclass initializes the class properties, the inherited superclass properties must be initialized too. To ensure this, the constructor of the subclass calls the superclass's constructor first, then goes on with its own constructor.

In your case, Z extends Y and Y extends X, so when you write new Z(), it calls constructor Z(), which implicitly calls Y() which calls X(). Now where is constructor X()? In java, when a class has no constructor, the compiler adds a default no-argument constructor to it. So your class X has a constructor X() already. This constructor will initialize x1, x2 and x3 to their default values, which is 0.

So the situation is, in main, you're calling Z(), Z() is calling Y() and Y() is calling X(). Ultimately the constructor X() initializes x3.