Symptoms

• Implementers leave methods empty or throw because they “do not apply.”
• Tests need huge fake objects for tiny call sites.

Refactor moves

• Split UserRepository into UserReader, UserWriter, or narrower ports (UserById, UserSearch) when call sites differ.
• In Dart, abstract classes and typedefs on function shapes both count as “interfaces.”

Example direction

abstract class UserLookup {
  User? byId(String id);
}

abstract class UserRegistration {
  void register(User draft);
}

• Admin console might implement both; read-only dashboard implements UserLookup only.

Practice tasks

• List methods on your largest interface and group them by caller; try merging only groups that always travel together.
• Replace a throw-stub in a fake with a smaller interface that omits that method.
• Add one typedef alias for a callback type that appears in three files.