Inheritance in JNI

Question

I'm trying to take an API that's written in Java and calling it from C++ using JNI.
My question deals with inherited types. For instance, in Java, we can do the following:
List list = new List();
String in = "me";
list.add(string);
String out = (String) list.get(0);
out.toString(out.substring(1));
When I get the string out of list, the type is Object, which I cast to String, and then I can do whatever I want just like it was a String (since that's what I put in).
So in my C++ code, a List such as the one above is given to me as the return value of a Java method. In other words, I didn't add anything to the List, it just comes back as a jobject that already has some other objects in it. Then I run the equivalent of list.get(0) using CallObjectMethod, which returns another jobject, of which I know the type. However when I try to run a method on that element of the list, I get a Java coredump and a seg fault.
I think what's happening is that the "type" of the jobject that gets returned from list.get(0) is java.lang.Object and when I try to run any object specific method, like String.substring(), JNI doesn't recognize the jobject as one of type String so it dies on me. Does anyone know how to "cast" a jobject? Or am I just completely off in my assessment of what is causing the problem?

Answer 1

Hi,
i face a problem in casting jobject in JNI. From Java layer i receive an array of super class objects. but in JNI layer i need to access the methods of subclass, for that i need to cast to subclass. in JNI there are methods like IsInstanceOf and IsAssignableFrom which can justify the validity of a casting, but there are no methods to cast as such. can some one help me sort it out.
Thanks in advance.