Dart: Final class

To close the type hierarchy, use the final modifier. This prevents subtyping from a class outside of the current library. Disallowing both inheritance and implementation prevents subtyping entirely. This guarantees:

• You can safely add incremental changes to the API.

• You can call instance methods knowing that they haven’t been overwritten in a third-party subclass.

Final classes can be extended or implemented within the same library. The final modifier encompasses the effects of base, and therefore any subclasses must also be marked base, final, or sealed.

// Library a.dart
final class Vehicle {
  void moveForward(int meters) {
    // ...
  }
}

// Library b.dart
import 'a.dart';

// Can be constructed
Vehicle myVehicle = Vehicle();

// ERROR: Cannot be inherited
class Car extends Vehicle {
  int passengers = 4;
  // ...
}

class MockVehicle implements Vehicle {
  // ERROR: Cannot be implemented
  @override
  void moveForward(int meters) {
    // ...
  }
}