When to use Final (from Slack)

when to use final.txt

Rule #1: if you can’t think of a use-case for extending, make the class final. At the same time, make everything private instead of protected.
Rule #2: if your class is final, all public methods must be part of an interface that people can mock
Rule #3: prefer small, targeted interfaces instead of a compound interface
Rule #4: listen to your users, hear their concerns, work with them to avoid opening a final class
Rule #5: if a user has a legitimate use-case for extending a final class (as opposed to wrapping it), open it up and make necessary methods protected. Keep fields private.