Question

In the following code, I am getting java.lang.InstantiationException

(Below is trimmed down code that compiles standalone – in my application I want to maintain an Enum->Class map, and on reading integer values from a file, instantiate appropriate class looking into the map).

How to get rid of the error? Is there a syntax problem? Must I use Interfaces? My understanding here is limited.

class Main {
    abstract class Base {
        Base() {};
        void print() {
            System.out.println("I am in Base");
        }
    }


    class D1 extends Base {
        D1() {};
        @Override 
        void print() {
            System.out.println("I am in D1");
        }
    }

    static Class<? extends Base> getMyClass() {
        return D1.class;
    }


    public static void main(String[] args) {
        try {
            Class<?> cc = getMyClass();
            Object oo = cc.newInstance();
            Base bb = (Base) oo; 
            bb.print();

        } catch (Exception ee) {
            System.out.println(ee);
        };
    }
};

Answer 1

Your code has two problems:

• Base and D1 are non-static inner classes. It means that they can access fields and methods of their declaring class (Main), therefore they should hold a reference to the instance of Main. Therefore their constructors have an implicit argument of type Main which is used to pass that reference. So, they don’t have no-args constructors and you should use a single-argument constructor instead:

  Object oo = cc.getConstructor(Main.class).newInstance(new Main()); 
  

  Alternatively, you can simply declare them as static, or declare them outside of Main – in this case they won’t be able to access member of Main and won’t require a reference to it.

• Constructor of D1 should be public. Otherwise you need to call setAccessible(true) to make it accessible for reflection.