Well my Viewer is basically my GUI. If I decide to create a new customer for example, how would I send it to the controller to add it to the database? ctrl.Add(new Customer(name, age)) or ctrl.Add(name, age) ?

Yes I understand that @Marjan I just thought it is widely used as we had studied it as a main part of design patterns, I am no more making this mistake :-)

SSD would mean System Sequence Diagram, I shall edit my question. I just thought that considering it is a question about an use case, mvc and design patterns, SSD will be understand as diagram and not a disc

Yeah, basically the whole later looks like this with the only difference the amount of parameters the methods have and forward. My main idea was, do the "good" programmers keep writing tests for every single class, however short/simple/easy it is. I have done the model so far, I will also do the db layer as all the magic happens there - I/O of data. Thank you for your answer :)

DBItem and DBSupplier are my classes for respectively Item and Supplier in the DB Layer. I also have IFItem and IFSupplier which are the interfaces.