Java HotSpot Virtual Machine

I am trying access a native library with an interface function which returns a base class pointer.

For example

C implementation *
Class Base {
int basevar;
}

Class Derived1 : public Base {
int derived1;
}

Class Derived2 : public Base {
int derived2;
}

Base myFunction(); // Function which returns base pointer. But it could be of type Derived1 or 2.

I am trying to access it From java using JNA. I have defined mapping files for all the types. But how can I resolve the type at runtime? Is there any possibility to identify this at runtime? (something like instanceof in Java).

Also when we specify the java function, we say it is of type base. But actually it could be of derived. Will JNA automatically do this coversion?
Any suggestions would be helpful.
Regards,
Boby Thomas