Do you actually create a DebitCard object to perform the operations on? It sounds like you are still calling methods against a CreditCard object.

You should consider trying to use full, descriptive names for your variables and methods, rather than cryptic abbreviations. I thought this was touched on in the coding conventions, but looking back I don't see it and must be thinking of a different document. Clear names that spell out the intent of a variable or method will help not only other readers of your code, but you as well. It's much easier to organize your thoughts when the intent of the code is clearly spelled out by variables and methods.