Step 1 - Product

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

  Product({
    required this.sku,
    required this.name,
    required this.price,
    this.quantity = 0,
  });
}

Step 2 - Inventory by SKU

class Inventory {
  final Map<String, Product> _bySku = {};

  void addProduct(Product p) {
    _bySku[p.sku] = p;
  }

  Product? find(String sku) => _bySku[sku];

  void restock(String sku, int n) {
    final p = _bySku[sku];
    if (p == null || n <= 0) return;
    p.quantity = p.quantity + n;
  }

  bool sell(String sku, int n) {
    final p = _bySku[sku];
    if (p == null || n <= 0 || p.quantity < n) return false;
    p.quantity = p.quantity - n;
    return true;
  }

  List<Product> get all => List.unmodifiable(_bySku.values);
}

void main() {
  Inventory inv = Inventory();
  inv.addProduct(Product(sku: 'A1', name: 'Notebook', price: 2.5, quantity: 10));
  inv.restock('A1', 5);
  inv.sell('A1', 3);
  print(inv.find('A1')?.quantity);
}

Step 3 - Stretch ideas

• Add double stockValue() summing price * quantity for all products.
• Prevent duplicate sku on addProduct (replace vs error vs ignore).

Practice tasks

• Add removeProduct(String sku) that deletes from the map.
• Add List<Product> lowStock(int maxQty) returning products where quantity <= maxQty.
• Print a table: sku, name, qty, line value (price * quantity).