Refactoring Bad Code Using SOLID Principles

public class CheckoutService {
  public void checkout(Cart cart, String paymentType) {
    double total = 0;
    for (var item : cart.items()) total += item.price * item.qty;
    if (paymentType.equals("CARD"))     total *= 1.029;
    else if (paymentType.equals("PP"))  total *= 1.034;
    var conn = DriverManager.getConnection("jdbc:mysql://...");
    var ps = conn.prepareStatement("INSERT INTO orders VALUES (?,?)");
    ps.setLong(1, cart.userId()); ps.setDouble(2, total); ps.execute();
    // raw SMTP send...
  }
}

class TotalCalculator { double total(Cart c) { /* ... */ } }
class FeeCalculator   { double withFees(double total, String type) { /* ... */ } }
class OrderRepository { void save(Order o) { /* ... */ } }
class OrderMailer     { void confirm(Order o) { /* ... */ } }

interface FeeStrategy { String type(); double apply(double amount); }

@Service
class CheckoutService {
  private final TotalCalculator totals;
  private final FeeCalculator fees;
  private final OrderRepository orders;
  private final OrderMailer mailer;
}

```java
@Service
@RequiredArgsConstructor
class CheckoutService {
  private final TotalCalculator totals;
  private final FeeCalculator fees;
  private final OrderRepository orders;
  private final OrderMailer mailer;