Generics | The Self-Improving Developer

main.dart

// Generics for Beginners playground.

void main() {
  var pants = Clothing(weight: 300);
  var shirt = Clothing(weight: 200);

  var satchel = Bag([pants, shirt]);
  print(satchel.runtimeType);

  var apple = Food(weight: 70, calories: 95);
  var bread = Food(weight: 100, calories: 265);

  var brownBag = Bag([apple, bread]);
  print(brownBag.runtimeType);

  // These work okay:
  print(satchel.weight);
  print(brownBag.weight);

  // But we would like the next line to work.
  // After all, we know that brownBag only
  // contains food.
  //print(brownBag.items.first.calories);

  // TODO(you): Use generics to make the above work.
}

class Bag {
  List<Item> items;

  Bag(this.items);

  /// Returns the weight of the whole bag,
  /// including its contents.
  int get weight {
    // The bag itself is 300g.
    int result = 300;
    for (var item in items) {
      result += item.weight;
    }
    return result;
  }
}

abstract class Item {
  /// Weight of the item in grams.
  int get weight;
}

class Clothing extends Item {
  int weight;

  Clothing({this.weight});
}

class Food extends Item {
  int weight;

  int calories;

  Food({this.weight, this.calories});
}