Seal your system'sboundaries·

// API types (you write these)
record CreateOrderRequest(Long userId, BigDecimal amount) {}
record OrderResponse(Long id, Long userId, String status) {}

// DB types (you write these too)
record OrderEntity(Long id, Long userId, BigDecimal amount, String status) {}

// The mapper (you write this)
class OrderMapper {
    OrderEntity toEntity(CreateOrderRequest req) {
        return new OrderEntity(null, req.userId(), req.amount(), "pending");
    }
    OrderResponse toResponse(OrderEntity e) {
        return new OrderResponse(e.id(), e.userId(), e.status());
    }
}

class OrderService {
    // BUG: userId and orderId are both Long — this compiles fine.
    User getUser(Long orderId) {
        return userRepository.findById(orderId);
    }
}

// Generated types — you write NOTHING.
// UserId, OrderId, OrderRow, OrderRowUnsaved,
// CreateOrderRequest, OrderResponse — all generated.

class OrderService {
    OrderResponse createOrder(CreateOrderRequest req) {
        OrderRow saved = orderRepo.insert(
            new OrderRowUnsaved(req.userId(), req.amount()), conn);
        return new OrderResponse(saved.id(), saved.userId(), saved.status());
    }

    // Won't compile: OrderId and UserId are different types.
    User getUser(OrderId orderId) {
        return userRepo.selectById(orderId);
        //                         ^^^^^^^
        //   error: required UserId, found OrderId
    }
}