Superclass variable can reference a subclass variable in java

/* JAVA program to understand superclass variable can reference a subclass variable */
/* www.codewindow.in */

class Box
{
    void volume()
    {
        return 100;
    }
}
class BoxWeight extends Box
{
    int height;
    int depth;
    int length;
    double breadth;
    BoxWeight(int a,int b, int c , double d)
    {
        height=a;
        depth=b;
        length=c;
        breadth=d;
    }
    double volume()
    {
        return height*length*height;
    }
    int weight()
    {
        return length*breadth*depth;
    }
}

class RefDemo
{
    public static void main(String args[])
    {
        BoxWeight weightbox = new BoxWeight(3, 5, 7, 8.37);
        Box plainbox = new Box();
        double vol;
        vol = weightbox.volume();
        System.out.println("Volume of weightbox is " + vol);
        System.out.println("Weight of weightbox is " +weightbox.weight);
        System.out.println();
        // assign BoxWeight reference to Box reference
        plainbox = weightbox;
        vol = plainbox.volume(); // OK, volume() defined in Box
        System.out.println("Volume of plainbox is " + vol);
        /* The following statement is invalid because plainbox does not define a weight member. */
        System.out.println("Weight of plainbox is " + plainbox.weight);
    }
}