Implementing Domain-Driven Design Patterns

// Sales Context: Customer = lead with revenue potential
class Customer { String id; BigDecimal lifetimeValue; List<Opportunity> opps; }

// Support Context: Customer = ticket holder
class Customer { String id; String tier; List<Ticket> openTickets; }

// Billing Context: Customer = payer
class Customer { String id; PaymentMethod method; BigDecimal balance; }

public class Order { // Aggregate Root
    private final OrderId id;
    private final CustomerId customerId; // Reference by ID
    private final List<OrderLine> lines = new ArrayList<>();
    private OrderStatus status;

    public void addLine(ProductId productId, int qty, Money price) {
        if (status != DRAFT) throw new IllegalStateException("Order locked");
        lines.add(new OrderLine(productId, qty, price));
    }
    public Money total() { return lines.stream().map(OrderLine::subtotal).reduce(Money.ZERO, Money::add); }
}

public record Money(BigDecimal amount, Currency currency) {
    public Money {
        if (amount == null || currency == null) throw new IllegalArgumentException();
    }
    public Money add(Money other) {
        if (!currency.equals(other.currency)) throw new IllegalStateException("Mismatch");
        return new Money(amount.add(other.amount), currency);
    }
}

public interface OrderRepository {
    Optional<Order> findById(OrderId id);
    List<Order> findByCustomer(CustomerId customerId);
    void save(Order order);
    void delete(OrderId id);
}

public record OrderPlaced(
    OrderId orderId,
    CustomerId customerId,
    Money total,
    Instant occurredAt
) implements DomainEvent {}

public class OrderFactory {
    public Order createFromCart(Cart cart, CustomerId customerId) {
        Order order = new Order(OrderId.generate(), customerId);
        cart.items().forEach(i -> order.addLine(i.productId(), i.qty(), i.price()));
        order.applyDiscounts(discountPolicy.eligible(customerId));
        return order;
    }
}

public interface Specification<T> { boolean isSatisfiedBy(T t); }

Specification<Order> eligibleForFreeShipping = o ->
    o.total().amount().compareTo(new BigDecimal("50")) >= 0
    && o.shippingAddress().country().equals("US");