NULL Pointer Exception from the class while accessing from EJB/ HttpSession

[Siva Ram]

Hi,
I have written a class(TestClass) which extends HashMap.
As HashMap already implements serializable interface. I am using this class in EJB trasactions which lead me to NULL Pointer exception.
When ever I am calling the ejb method , I am getting the NULL Pointer exception. Then I called the dao without ejb. It works fine. But the problem is when I set the particular dao output (i.e TestClass object) to HttpSession, it again throws NULL Pointer exception.
It is throwing NULL Pointer exception in put method of the Hashtable.
Can any one please help me to solve this issue please.
Thanks & Regards,
Sivaram