sendUserRegistrationConfirmation(User user); } public class OrderService { private final NotificationService notificationService; public void processOrder(Order order) { // 注文処理... notificationService.sendOrderConfirmation(order); } } public class UserService { private final NotificationService notificationService; public void registerUser(User user) { // ユーザー登録処理... notificationService.sendUserRegistrationConfirmation(user); } }