Phase 11 of 20 · Topic 11.5

Loose Coupling & Dependency Inversion Principle (DIP)

1Concept

The Dependency Inversion Principle (SOLID) states: High-level modules should not depend on low-level modules; both should depend on abstractions. Designing components against interfaces rather than concrete classes achieves loose coupling and seamless testability (mocking).

2Architecture Diagram

Tightly Coupled:   [ OrderService ] ---> [ PostgresDatabase ] (Rigid! Cannot swap or mock)
Loosely Coupled:   [ OrderService ] ---> [ DataRepository Interface ] <--- [ Postgres / Mongo / Mock ]

3Code Example

Core Java
public class DependencyInversionDemo {
    // Abstraction contract
    interface MessageSender {
        void sendMessage(String recipient, String message);
    }

    // Concrete Low-level implementation
    static class SmtpMailSender implements MessageSender {
        public void sendMessage(String recipient, String message) {
            System.out.println("SMTP mail sent to " + recipient + ": " + message);
        }
    }

    // High-level service depending strictly on abstraction
    static class NotificationCoordinator {
        private final MessageSender sender;

        // Injected via constructor (Dependency Injection)
        public NotificationCoordinator(MessageSender sender) {
            this.sender = sender;
        }

        public void notifyUser(String user) {
            sender.sendMessage(user, "Security Alert: New login from unknown IP");
        }
    }

    public static void main(String[] args) {
        MessageSender emailSender = new SmtpMailSender();
        NotificationCoordinator coordinator = new NotificationCoordinator(emailSender);
        coordinator.notifyUser("admin@enterprise.com");
    }
}

4Expected Output

SMTP mail sent to admin@enterprise.com: Security Alert: New login from unknown IP

5Key Takeaways

  • ✓Always code to interfaces, not concrete implementations.
  • ✓Dependency injection facilitates frictionless unit testing using mock frameworks (Mockito).
  • ✓Loose coupling minimizes regression blast radius when swapping infrastructure components.