4.java

// This program uses inheritance to extend Box.
class Box {
    double width;
    double height;
    double depth;

    // construct clone of an object
    Box(Box ob) { // pass object to constructor
        width = ob.width;
        height = ob.height;
        depth = ob.depth;
    }

    // constructor used when all dimensions specified
    Box(double w, double h, double d) {
        width = w;
        height = h;
        depth = d;
    }

    // constructor used when no dimensions specified
    Box() {
        width = -1; // use -1 to indicate
        height = -1; // an uninitialized
        depth = -1; // box
    }

    // constructor used when cube is created
    Box(double len) {
        width = height = depth = len;
    }

    // compute and return volume
    double volume() {
        return width * height * depth;
    }
}

// Here, Box is extened to include weight.
class BoxWeight extends Box {
    double weight; // weight of box

    // constructor for BoxWeight
    BoxWeight(double w, double h, double d, double m) {
        width = w;
        height = h;
        depth = d;
        weight = m;
    }

    double getDensity() {
        return weight / volume();
    }
}

class RefDemo {
    public static void main(String args[]) {
        BoxWeight weightbox = new BoxWeight(3, 5, 7, 8.37);
        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("Density of weightbox is " + weightbox.getDensity());
        System.out.println();

        Box plainbox = new BoxWeight(3, 5, 7, 8.37);

        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);
    }
}