It must have enough methods to get the behavior you want, that doesn't always mean all subtypes. Assuming you're running a version of Java that supports defaul

It must have enough methods to get the behavior you want, that doesn't always mean all subtypes. Assuming you're running a version of Java that supports default implementations, which may often be used to avoid breaking the interface. Sum types are a different story, as you would have to dive into existing code.