CODEWITHSUNDEEP
javainner-classes
Dineshkumar
Dineshkumar

Reputation: 4245

Inheriting InnerClass in java

public class InnerClass {
    class Inner
    {
        public void method()
        {
            System.out.println("Innerclass");
        }
    }
}

class Sample extends InnerClass.Inner
{
    public static void main(String [] arg)
    {
        Sample s = new Sample(new InnerClass());
        s.method();

    }

//why is this mandatory???

    Sample(InnerClass i) {
        i.super();
    }

    @Override
    public void method() {
        System.out.println("derived class");
    }
}

when i make a class that derives from an innerclass (Innerclass.Inner) default constructor doesn't works. later i came to know that it requires to include a constructor taking Enclosing class reference why is it so?

Upvotes: 1

Views: 120

Answers (2)

Sebastian Redl
Sebastian Redl

Reputation: 72215

Because non-static inner classes have an implicit member that points back to their outer class, and you can't create an instance of the inner class without giving it that pointer. If you directly create an instance of an inner class, you have to use new outer.Inner() (or it might be outer.new Inner(), I can never remember). But Sample isn't an inner class, it just inherits one, so the outer instance must be passed in its constructor to the base constructor. Thus, it needs to have some instance of outer available, or create it itself.

Upvotes: 0

NG.
NG.

Reputation: 22914

Non static inner classes in Java have an implicit reference to the enclosing instance. You can solve your problem with:

public class InnerClass {
    static class Inner // can make it public too
    {
        public void method()
        {
             System.out.println("Innerclass");
        }
    }
}

Just don't expect to be able to call any methods on InnerClass without a specific instance.

Upvotes: 2

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