Class members vs object variables of subclass

Question

Below is the code snippet and its output -

class Box{
        int height = 1;
        int length = 1;
        int width = 1;

        Box(int h, int l, int w){
                this.height = h;
                this.length = l;
                this.width = w;
        }

        int volume(){
                return this.height * this.length * this.width;
        }

}

class BoxWeight extends Box{
        int height = 99;
        int length = 99;
        int width = 99;
        int mass;

        BoxWeight(int h, int l, int w, int m) {
                super(h, l, w);
                this.mass = m;
        }

        int volume(){
                return this.height * this.length * this.width;
        }
}
public class BoxDemo {
        public static void main(String[] args) {
                Box b = new Box(10, 20, 30);
                BoxWeight bw = new BoxWeight(40, 50, 60, 10);
                System.out.println(bw.height);
                System.out.println(bw.length);
                System.out.println(bw.width);
                //b = bw;
                //System.out.println(b.volume());

        }

}

Output

99
99
99

Here i am not able to get why object bw is printing values initialized as class members. Why object bw is not holding values assigned through constructor?

actually ultimate aim is not to print 40, 50, 60 , i just wanted to know whats happening behind the scene here. If anyone can put here some good read regarding this, it can be helpful. Which java mechanism working here? — anil funde, Apr 14 '15 at 14:43
when in BoxWeight constructor ""BoxWeight(40, 50, 60, 10)** is called which again calls **super** to initialize height, width, length i thought all initialization using super will be done on **this** means BoxWeight object which in turn will have values 40, 50, 60. — anil funde, Apr 14 '15 at 14:59

score 4 · Answer 1 · answered Apr 14 '15 at 12:25

The members of BoxWeight hide the members of Box, so you don't see the values you passed to the Box constructor.

Just remove this members from BoxWeight :

        int height = 99;
        int length = 99;
        int width = 99;

It also makes no sense to have the exact same implementation of volume() in both classes.

score 1 · Answer 2 · answered Apr 14 '15 at 12:36

Because BoxWeight defines it's own length, width, and height, the corresponding fields in Box are hidden. You can either remove the fields from BoxWeight, or cast to a Box as follows:

public static void main(String[] args) {
    Box b = new Box(10, 20, 30);
    BoxWeight bw = new BoxWeight(40, 50, 60, 10);
    System.out.println(((Box) bw).height);
    System.out.println(((Box) bw).length);
    System.out.println(((Box) bw).width);
}

Prints:

40
50
60

score 0 · Answer 3 · answered Apr 14 '15 at 12:27

0

That's how Inheritance is working. The last @Override counts. In this case it is the BoxWeight.

answered Apr 14 '15 at 12:27

Murat Karagöz

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Class members vs object variables of subclass

3 Answers3