intro_007.1.dart

void main() {
  var breakfast = Breakfast(
    tea: Tea(
      milk: Milk(),
    ),
  );

  print('$breakfast was ${breakfast.getCalories()} calories');
}

abstract class Meal {
  int getCalories();
}

class Breakfast extends Meal {
  Tea tea;

  Breakfast({this.tea});

  @override int getCalories() {
    if (tea != null) return tea.getCalories();
    return 0;
  }

  @override
  String toString() => 'Breakfast';
}

class Tea extends Meal {
  Milk milk;

  Tea({this.milk});
  
  @override int getCalories() {
    if (milk != null) return milk.getCalories();
    return 0;
  }

  @override
  String toString() => 'Tea';
}

class Milk extends Meal {
  
  // default constructor Milk() is added by compiler
  
  @override int getCalories() => 20;

  @override
  String toString() => 'Milk';
}