The performance difference between interfaces and abstract classes depends on the implementation. You can have several member definitions in an abstract class or an interface.

Learn the difference between classes and interfaces once and for all, then get started declaring, implementing, and extending interfaces in your Java programs Topics Spotlight: New Thinking about ...

I suppose I could write some unreadable reflection code to extract the class's TypeName, but really, the programmer who creates the class should decide what counts as "KeyData" for the class.