FINAL if not designed for inheritance

Make classes that are not explicitly designed for inheritance FINAL.

When designing class cooperation, your first choice should be composition, not inheritance. Enabling inheritance is not something that should be done lightly, as it requires you to think about things like PROTECTED vs. PRIVATE and the Liskov substitution principle, and freezes a lot of design internals. If you didn’t consider these things in your class design, you should thus prevent accidental inheritance by making your class FINAL.

There are some good applications for inheritance, of course, for example the design pattern composite. Business Add-Ins can also become more useful by allowing sub-classes, enabling the customer to reuse most of the original code. However, note that all of these cases have inheritance built in by design from the start.

Unclean classes that don’t implement interfaces should be left non-FINAL to allow consumers mocking them in their unit tests.