Dart: Sealed class

To create a known, enumerable set of subtypes, use the sealed modifier. This allows you to create a switch over those subtypes that is statically ensured to be exhaustive.

The sealed modifier prevents a class from being extended or implemented outside its own library. Sealed classes are implicitly abstract.

• They cannot be constructed themselves.

• They can have factory constructors.

• They can define constructors for their subclasses to use.

Subclasses of sealed classes are, however, not implicitly abstract.

The compiler is aware of any possible direct subtypes because they can only exist in the same library. This allows the compiler to alert you when a switch does not exhaustively handle all possible subtypes in its cases:

sealed class Vehicle {}

class Car extends Vehicle {}

class Truck implements Vehicle {}

class Bicycle extends Vehicle {}

// ERROR: Cannot be instantiated
Vehicle myVehicle = Vehicle();

// Subclasses can be instantiated
Vehicle myCar = Car();

String getVehicleSound(Vehicle vehicle) {
  // ERROR: The switch is missing the Bicycle subtype or a default case.
  return switch (vehicle) {
    Car() => 'vroom',
    Truck() => 'VROOOOMM',
  };
}