Demo :: DI

Email.java

public class Email {

	public void send() {		
	}
}

Email2.java

public class Email implements MessageService {

	public void send() {		
	}
}

MessageService.java

public interface MessageService {
	
	public void send();

}

Notification.java

public class Notification {
	
	Email email;
	
	public Notification() {
		email = new Email();
	}
	
	public void notifyPromotion() {
		email.send();
	}

}

Notification2.java

public class Notification {
	
	MessageService messageService;
	
	public Notification() {
		messageService = new Email();
	}
	
	public void notifyPromotion() {
		messageService.send();
	}

}

Notification3.java

public class Notification {
	
	private MessageService messageService;
	
	public Notification(MessageService messageService) {
		this.messageService = messageService;
	}
	
	public void notifyPromotion() {
		messageService.send();
	}

}

Notification4.java

public class Notification {

	private MessageService messageService;

	public void setMessageService(MessageService messageService) {
		this.messageService = messageService;
	}

	public void notifyPromotion() {
		if (this.messageService != null) {
			messageService.send();
		}
	}

}

Notification5.java

public class Notification {

	public void notifyPromotion(MessageService messageService) {
		messageService.send();
	}

}