Implementing Domain-Driven Design Patterns

public record Email(String value) {
    public Email {
        Objects.requireNonNull(value);
        if (!value.matches("^[^@]+@[^@]+\\.[^@]+$")) throw new IllegalArgumentException();
        value = value.toLowerCase(Locale.ROOT);
    }
}

public class Order {  // aggregate root
    private final OrderId id;
    private final List<OrderLine> lines = new ArrayList<>();
    public void addLine(ProductId p, int qty) {
        if (lines.size() >= 100) throw new IllegalStateException("max 100 lines");
        lines.add(new OrderLine(p, qty));
    }
    public List<OrderLine> lines() { return List.copyOf(lines); }
}

public class PricingService {
    public Money price(Order o, Customer c, DiscountPolicy p) {
        return p.apply(o, c);
    }
}

public class OrderFactory {
    public Order create(CustomerId c, List<CartItem> items, IdGenerator idGen) {
        var order = new Order(new OrderId(idGen.next()), c);
        items.forEach(i -> order.addLine(i.productId(), i.qty()));
        return order;
    }
}