What is dependency injection and how does it relate to design patterns? Dependency injection क्या है और यह design patterns से कैसे संबंधित है?

Dependency injection (DI) is a technique where an object's dependencies are provided from outside rather than created internally, achieving loose coupling and following the Dependency Inversion Principle.

// WITHOUT dependency injection - tight coupling
class EmailService {
    public void send(String message) {
        System.out.println('Email: ' + message);
    }
}

class NotificationServiceOld {
    private EmailService emailService = new EmailService();  // hardcoded, tightly coupled

    public void notify(String message) {
        emailService.send(message);
    }
}
// Problem: NotificationServiceOld can ONLY ever use EmailService,
// hard to test (can't substitute a mock), hard to swap implementations

// WITH dependency injection - loosely coupled via an interface
interface MessageService {
    void send(String message);
}

class EmailServiceImpl implements MessageService {
    public void send(String message) {
        System.out.println('Email: ' + message);
    }
}

class SmsServiceImpl implements MessageService {
    public void send(String message) {
        System.out.println('SMS: ' + message);
    }
}

class NotificationService {
    private final MessageService messageService;

    // Constructor injection - dependency provided from OUTSIDE
    public NotificationService(MessageService messageService) {
        this.messageService = messageService;
    }

    public void notify(String message) {
        messageService.send(message);
    }
}

// Client decides WHICH implementation to inject
NotificationService emailNotifier = new NotificationService(new EmailServiceImpl());
NotificationService smsNotifier = new NotificationService(new SmsServiceImpl());

emailNotifier.notify('Hello');  // Email: Hello
smsNotifier.notify('Hello');    // SMS: Hello

// Easy to test with a mock implementation
class MockMessageService implements MessageService {
    public void send(String message) {
        System.out.println('Mock sent: ' + message);  // no real email/SMS sent in tests
    }
}

// Frameworks like Spring automate this wiring using @Autowired,
// managing object creation and injection so you rarely write 'new' manually