How about something like Final by Default

[bangarharshit]

Probably a check which checks if the class is not marked with @Open or abstract should not have any subclasses.

On a related note, it will be a great thing to have FinalByDefault (similar to NonNullByDefault) baked in the type system via EP check or checker framework.

[JakeWharton]

If it's not @Open or abstract though then what is it? Isn't final all that's left?

[bangarharshit]

Fair point. Just avoiding the keyword final. It should be declared @Open or abstract. In the current check Non-final, Non-abstract classes without the @Open annotation will be marked with an error.. How about just treating them as final. All things are final by default and you have to mark @Open to make it subclassable.

[JakeWharton]

That still allows their subclassing by downstream libraries. I suppose if the type isn't effectively public then perhaps the check shouldn't apply?

[bangarharshit]

Oh. Didn't think of this use case.