how to do unit testing using the junit

I'm having a hard time creating a cascading drop down list that have a relationship, but when I remove the the relationship It works fine.. I have 3 links the vendor master, expense master and credit card page, The Vendor Master and Expense Master are in relationship when I create Vendor transaction it have a expense category field where in database it is the (ExpenseID) and link to the table of the expense table..

It works fine creating the vendor transaction but my problem here is in Credit Card Page where when I select the vendor it will populate the field of expense like in my statement above it have some kind of relationship. Thank You