Question

We can access the Super Class methods which consists of operations on private data members and print the results.But why can’t I print the private data members of Super Class with the SubClass object calling them in my main function? Someone please explain me.

Here is the example below.

class SuperClass1
{
    private int a;
    private int b;

    SuperClass1(int p,int q)
    {
        a=p;
        b=q;
    }
    SuperClass1(SuperClass1 obj)
    {
        a=obj.a;
        b=obj.b;
    }
    SuperClass1()
    {
        a=-1;
        b=-1;
    }

    int Vol()
    {
        return a*b;
    }
}

class SubClass1 extends SuperClass1
{
    int c;

    SubClass1(int p,int q,int r)
    {
        super(p,q);
        c=r;
    }
    SubClass1(SubClass1 obj)
    {
        super(obj);
        c=obj.c;
    }
    SubClass1()
    {
        super();
        c=-1;
    }
}


public class Super 
{
    public static void main(String[] args) 
    {
        SubClass1 obj1=new SubClass1();
        //System.out.println("The values of obj1 are:"+obj1.a+""+obj1.b+""+obj1.c);
        int vol=obj1.Vol();
        System.out.println("The volume is :"+vol);
    }
}

Answer 1

security and encapsulation

The superclass is letting its subclasses use only the public and protected methods/fields.

This allows the designer of the superclass to change the implementation of these methods if he sees it better, without breaking the subclass’s correctness.

A text book example is a complex number class.

The programmer using this class only needs its functionality, he doesn’t care if the implementation is with imaginary and real fields or with radius and theta fields [two distinct ways to represent complex number].

It allows the designer of the ComplexNumber class more freedom if he wants to change the class in later versions, and it also allows the user less worries: he doesn’t need to take care for all the details, some are being taken care of for him.

Bonus: note you can break this behavior and access private fields and methods by using reflection – but when you do so – all bets are off, and you do it on your own responsibility.