You can always make input validation errors a checked exception with your own exception type. It'll accomplish what you're trying to accomplish. It's not perfectly clean because it'd be nice if you could stick it under the "IllegalArgumentException" part of the inheritance tree, but it does what you're asking for.
Right but if you derive it from Error it isn't checked and same as if it is derived from RuntimeException. Deriving from Exception is checked but then you aren't fitting in with the hierarchy that pron is recommending, or at least what I think they are recommending, which is why I asked for clarification.
Internal errors are bugs or other conditions that the application is not expected to handle (other than, say, restart the thread a-la Erlang). Input validation is not an internal error. IllegalArgumentException is meant to represent bugs, and so an internal error, and so it is unchecked. It does not represent input validation errors.
