Step 1 - User

class User {
  final String id;
  final String name;
  final String email;

  const User({
    required this.id,
    required this.name,
    required this.email,
  });
}

void main() {
  const User u = User(id: '1', name: 'Asha', email: 'a@example.com');
  print(u.name);
}

Step 2 - Product

class Product {
  final String sku;
  final String name;
  final double price;

  const Product({
    required this.sku,
    required this.name,
    required this.price,
  });
}

Step 3 - OrderItem

class OrderItem {
  final String productSku;
  final int quantity;
  final double unitPrice;

  const OrderItem({
    required this.productSku,
    required this.quantity,
    required this.unitPrice,
  });

  double get lineTotal => unitPrice * quantity;
}

Step 4 - Wire in main

void main() {
  const User u = User(id: '1', name: 'Asha', email: 'a@example.com');
  const Product p = Product(sku: 'SKU-1', name: 'Mug', price: 9.99);
  const OrderItem line = OrderItem(
    productSku: p.sku,
    quantity: 2,
    unitPrice: p.price,
  );
  print('${u.name} buys ${line.quantity} for ${line.lineTotal}');
}

Practice tasks

• Add lightweight validation in factories or constructors (reject empty sku, non-positive quantity) using ArgumentError or early returns, still without mutable fields.
• Add String describe() on each type returning one readable line.
• Keep OrderItem independent of Product type (sku + prices only) vs hold a Product reference; try both and note tradeoffs.