If your interface is going to be implemented by just one class, I'd offer to skip interface - it reduces amount of code. If you still need to pass, as a parameter, a class A in order to test a class B, you can instead pass class C inherited from A, with all necessary methods overloaded.

If you just create a class A in java, no interfaces involved, class A will have its methods virtual. I'm not sure what do you mean by interface/implementation divide.