If you have a sufficiently statically typed language then the is-a concern goes away -- certainly in the example you gave, since the compiler/linker knows to look for a `GetNotifier()` that returns a `Notifier`. Now, you might still want to know whether the notifier you got satisfies other traits than just those of `Notifier`, but you do that by using a type that has those traits rather than `Notifier`, and now you have little need for `instanceof` or similar operators. (You still might want to know what kind of thing you got because you might care about semantics that are not expressed in the interface. For example you might care to know whether a logger is "fast" as in local or "slow" as in remote, as that might cause you to log less verbosely to avoid drops or blocking or whatever. But the need for this `instanceof` goes down dramatically if you have interfaces and traits.)