Not able to create an object of subclass in separate class in eclipse

Question

I'm learning inheritance in java. While writing small code where I'm learning we cannot access private members of superclass in subclass.

Here is my code:

class A {
    int i;
    private int j;

    void setij(int x, int y) {

        i = x;
        j = y;
    }

    class B extends A {

        int b;

        void sum() {

            b = i + j;
        }
    }

When I create a new class in eclipse, am not able to create an object of sublcass B in class A.

class mainclass{

            public static void main(String args[]){
                 B object = new B(); ----error


            }
        }

    }

Error says class B needs to be created.

May I know why is happening ..? Its happening not a problem but I want to solve and understand the logic why it was happened.

Thanks

Have you tried `A.B object = new A.B();`? `B` is a nested class (maybe it also needs to be declared as a `static class`) — Oneiros, Jun 26 '17 at 12:26
Did you get an Errormessage? In my Point of view it seems, as if you forgot a closing bracket. Not sure if that's the reason though. — T.Semmler, Jun 26 '17 at 12:29

Darshan Mehta · Accepted Answer · 2017-06-26T13:49:02.133

2

That's because class B is present inside class A, it needs to be created outside of A, e.g.:

class A {
    int i;
    private int j;

    void setij(int x, int y) {

        i = x;
        j = y;
    }   
}

class B extends A {

     int b;
     void sum() {
        b = i + j;
     }
 }

With this code, class B (and A) will be accessible from all the classes from inside the package. If you want these classes to be accessible from outside the current package then you need to create these classes in separate files and mark them as public.

Here's more on access modifiers for classes.

Upate

If you want to define class B as an inner class in A then you will need the instance of enclosing class (A in our case) to access B, e.g.:

class A {
    int i;
    private int j;

    void setij(int x, int y) {

        i = x;
        j = y;
    }   

    class B extends A {

        int b;
        void sum() {
           b = i + j;
        }
    }
}

Now, to instantiate B, you need to do the following:

A a = new A();
B b = a.new B();

And of course, same rules as access modifier will apply to inner class instance (except the fact that inner class will be able to access private members of enclosing class),

edited Jun 26 '17 at 13:49

answered Jun 26 '17 at 12:25

Darshan Mehta

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Yes, or declared static – Maurice Perry Jun 26 '17 at 12:26
@MauricePerry yes, exactly! – Darshan Mehta Jun 26 '17 at 12:28
@DarshanMehta...but am creating main class in same package...!! – LearnJava Jun 26 '17 at 12:34
Errr... no: this will not work because j is private! – Maurice Perry Jun 26 '17 at 12:38
Here are the screen shots http://prntscr.com/fob2zs and https://prnt.sc/fob2ux – LearnJava Jun 26 '17 at 12:38
@LearnJava your Screenshot confirms this answer. `B` is an inner class of `A` and in this way it is not accessible from any other class that is not `A`. Move `B` to it's own file. – André Stannek Jun 26 '17 at 12:41
@AndréStannek...so you mean If I do like this, superclass object must be created before i create an instance of sublcass..? What make difference if I create my inner class B in different file...? After doing this what make this to do where am getting error..? – LearnJava Jun 26 '17 at 12:46
@LearnJava this has nothing to do with superclass and subclass. It's about inner and outer class. You are mixing up two entirely different concepts. `B` isn't accessible if it's a non static inner class. If you move it to a new file, it's not an inner class anymore but still a subclass of `A`. – André Stannek Jun 26 '17 at 12:50
@AndréStannek..and what if B was an static inner class..? – LearnJava Jun 26 '17 at 12:54
@LearnJava that should also work but it would be bad style. – André Stannek Jun 26 '17 at 12:59
@LearnJava do you want `B` to be an inner class or a separate class? It's not specified in the question, however, I can update the answer if that's what the requirement is. – Darshan Mehta Jun 26 '17 at 13:02
@DarshanMehta...Inner class – LearnJava Jun 26 '17 at 13:30
@LearnJava updated the answer – Darshan Mehta Jun 26 '17 at 13:49
@DarshanMehta...what it signifies B b = a.new B(); Instance of class B resides inside Class A instance..? If yes ..so it means outer class instance producing inner class instance...correct..? Parent---->Child – LearnJava Jun 26 '17 at 14:00
@LearnJava yes, that's correct. Outer class instance acts like a handle to access inner class instance. – Darshan Mehta Jun 26 '17 at 14:03

score 1 · Answer 2 · answered Jun 26 '17 at 12:41

This is because an instance of class B must relate to an instance of class A. So you must first create an instance of class A, then the instance of class B:

    A a = new A();
    a.setij(1,2);
    B b = a.new B();
    b.setij(3,4);
    b.sum();
    System.out.println(b.b);

(the result is 5 in case you were wondering)

Not able to create an object of subclass in separate class in eclipse

2 Answers2