Without seeing your code, it's not possible to know exactly.  Also, to
answer, it's important to know how familiar you are with Java
compile-time and runtime types, and casting.  

This means that somewhere in your code your try to cast an object to a
type to which the object can not be casted.

Object o = new Integer(0);
Integer i = (Integer) o; // Working cast
String s = (String) o; // ClassCastException

Regards,

Bart