one constructor in subclass is working while the default is not

The result of the default constructor is not returning.The constructor with parameter in subclass is working but the default is not.

 subxy() {
    System.out.println("the value of z is" + z);
}

This part is not working. What's the reason? The full code with output is given below.

class superxy {
    double x;
    double y;

    superxy() {
        x = 8;
        y = 9;
        System.out.println("the value of x and y is" + x + " " + y);
    }
}

class subxy extends superxy {
    double z;

    subxy() {
        System.out.println("the value of z is" + z);
    }

    subxy(double a, double b, double c) {
        x = a;
        y = b;
        z = c;
        double result;
        result = x + y + z;
        System.out.println(result);
    }

}

public class xy {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        subxy mybox1 = new subxy(2, 3, 4);

    }

}

the output is:

the value of x and y is8.0 9.0
9.0