visitor.dart

abstract class PlanetVisitor {
  void visit({Planet planet});
}

abstract class Planet {
  void accept({PlanetVisitor visitor});
}

class MoonJedah extends Planet {
  @override
  void accept({PlanetVisitor visitor}) => visitor.visit(planet: this);
}

class PlanetAlderaan extends Planet {
  @override
  void accept({PlanetVisitor visitor}) => visitor.visit(planet: this);

}

class PlanetCoruscant extends Planet {
  @override
  void accept({PlanetVisitor visitor}) => visitor.visit(planet: this);
}

class PlanetTatooine extends Planet {
  @override
  void accept({PlanetVisitor visitor}) => visitor.visit(planet: this);
}

class NameVisitor extends PlanetVisitor {
  String name = "";

  NameVisitor([this.name]);

  @override
  void visit({Planet planet}) {
    if (planet is PlanetAlderaan) {
      name = "Alderaan";
    } else if (planet is PlanetCoruscant) {
      name = "Coruscant";
    } else if (planet is PlanetTatooine) {
      name = "Tatooine";
    } else if (planet is MoonJedah) {
      name = "Jedah";
    }
  }
}

main(List<String> arguments) {
  final planets = <Planet>[new PlanetAlderaan(), new PlanetCoruscant(), new PlanetTatooine(), new MoonJedah()];
  final names = planets.map<String>((planet) {
    final visitor = new NameVisitor();
    planet.accept(visitor: visitor);
    return visitor.name;
  });

  names.forEach((name) => print(name));
}