Android :: How Java Generate Signatures For Methods

I have an Java class with a static final method getAll:

CODE:..............

Now I want to hand in null as a value for the where statemant so that it will just be ignored later on in the code. Anyway in the testcase for this method I have: Vector<Category> categories = Category.getAll(context, null);

Which then in turn gives me a NoSuchMethodError. I don't know exactly why it does that. The only thing I could imagine is that the null I hand in would not match the signature of the above method. But how can I overcome this? I already thought of overloading. But this would just end in rewriting most of the code. At least when I do it, how I think.

This is the stack trace I get:

CODE:.......................