使用super关键字从子类访问超类成员

Question

I have a subclass and a superclass. In the subclass when I want to retrieve the values of the super class with super.i and super.one it shows zero . Why? Also when I extend a superclass method to a subclass is there absolute necessity to call the superclass member function using super keyword?

public class Inherit{
public static void main(String args[])
{
System.out.println("Hello Inheritance!");
Date now = new Date();
System.out.println(now);
Box hello = new Box(2,3,4);
BoxWeight hello_weight = new BoxWeight(2,5,4,5);
hello.volume();
hello_weight.volume();
Box hello_old = hello;
hello = hello_weight;
//hello.showValues();
hello.show();
hello_old.show();
hello = hello_old;
hello.show();
hello.setValues(7,8);
hello_weight.setValues(70, 80);
hello.showValues();
hello_weight.showValues();
}
}
class Box{
int width, height, depth, i, one;
static int as=0;
Box(int w, int h, int d)
{
    ++as;
    width = w;
    height = h;
    depth = d;
    }
    void setValues(int a, int k)
    {
        i = k;
        one = a;
        System.out.println("The values inside super are : " + i +" " + one +" " +  as);
        }
        void showValues()
        {
            System.out.println("The values of BoxWeight : " + i +" " + one);
            //System.out.println("The superclass values : "+ super.i + " " + super.one);
            }
void volume()
{
System.out.println("Volume : " + width*height*depth);
}
void show()
{
    System.out.println("The height : " + height);
    }
}
class BoxWeight extends Box{
int weight,i,one;
void volume()
{
    System.out.println("Volume and weight : " + width*height*depth +" "+ weight);
    }
    void setValues(int a, int k)
    {
        i = k;
        one = a;
        }
        void showValues()
        {
            System.out.println("The values of BoxWeight : " + i +" " + one);
            System.out.println("The superclass values : "+ super.i + " " + super.one);
            }
BoxWeight(int w, int h, int d, int we)
    {
        super(w,h,d);
        weight = we;
        }
}

Answer 1

because you have not initialized one, so by default it gets value as zero.

hello_weight 

is object of Box_Weight class, and when you call setValues of that class, one of this class is initailized, and super class one is shadowed . So super class one is still zero.

And one is not initialized in constructor.

Answer 2

You don't need the super keyword to access members of the parent class. What you do need, however, is to have proper scope / visibility.

If your parent class has fields that are protected as opposed to private , then only members of the subclass can see them.

Answer 3

default scope of a variable is package private. So if you want to access the parent class variables than make is protected or place child and parent both in same packages.

In your case the int width, height, depth, i, one; variables are package private so if your Sub class is not inside the same package than it can't access. Hence declare these as protected .