The document discusses implementing the singleton pattern to broadcast messages to different types of notice boards in a flexible way. It describes evolving the implementation from a basic singleton to support multiple notice board types by applying principles of abstraction, dependency injection and the factory pattern. This allows creating notice board, broadcaster and messenger objects dynamically based on the board type while maintaining loose coupling between components.

1. Singleton

6. Case - Study

7. Notice Board detailed Domain

8. OnSubmit for send Message Static binding Singleton as Evil

9. Singleton public class MessageBroadCaster { private static final MessageBroadCaster INSTANCE = new MessageBroadCaster(); private MessageBroadCaster() {} public static MessageBroadCaster getInstance () { return INSTANCE; } public void broadCast(final String[] messages) { final NoticeBoard board = NoticeBoard.getInstance(); int i = 0; final Message[] todayMessages = new Message[messages.length]; for(final String message : messages) { todayMessages[i++] = new Message(message); } board.show(todayMessages); } } public class NoticeBoard { private static final NoticeBoard noticeBoardInstance = new NoticeBoard(); private NoticeBoard () {} public static NoticeBoard getInstance () { return noticeBoardInstance; } public void show(final Message[] messages){ for(Message msg : messages) msg.display(); } } Global Variables are many … Class operation

12. Abstraction

13. public class MessageBroadCaster{ private NoticeBoard noticeBoard; public MessageBroadCaster(NoticeBoard noticeBoard) { this.noticeBoard = noticeBoard; } public void broadCast(final String[] messages) { int i = 0; final Message[] todayMessages = new Message[messages.length]; for(final String message : messages) { todayMessages[i++] = new Message(message); } noticeBoard.show(todayMessages); } } Abstraction class Simple implements NoticeBoard{ public void show(Message[] messages) { System.out.println("Simple Display ..."); for(Message message: messages) { System.out.println(message.getContent()); } } } Instance operation, with good abstraction

14. Modules Sequence Simple Messenger Simple Message Broadcaster Simple Notice Board sends broadcast Simple LCD Messenger LCD Message Broadcaster LCD Notice Board sends broadcast LCD Electronic Messenger Electronic Message Broadcaster Electronic Notice Board sends broadcast Electronic This is not a

15. How to create Messenger for other modules like LCD,Electronic ..etc public class Messenger { private MessageBroadCaster messageBroadCaster; public Messenger(MessageBroadCaster messageBroadCaster) { this.messageBroadCaster = messageBroadCaster; } public void send(String[] messages) { messageBroadCaster.broadCast(messages); } public static void main(String[] args) { NoticeBoard simpleNoticeBoard = new Simple (); MessageBroadCaster simpleMessageBroadCaster = new MessageBroadCaster(simpleNoticeBoard) ; Messenger simpleMessenger = new Messenger(simpleMessageBroadCaster); simpleMessenger.send (new String[] { "Welcome to Singleton" , "Town hall meet" }); } } Simple Module Simple Notice Board flow or Module has been tested.

17. Program Idiom public class NoticeBoardFactory { public NoticeBoard createNoticeBoard(String type) { if("Simple".equals(type)) { return new Simple(); } else if("LCD".equals(type)){ return new LCD(); } else if("Electronic".equals(type)) { return new Electronic(); } else { return null; }}} public static void main(String[] args) { NoticeBoardFactory boardFactory = new NoticeBoardFactory(); NoticeBoard simpleNoticeBoard = boardFactory.createNoticeBoard("Simple"); MessageBroadCaster simpleMessageBroadCaster = new MessageBroadCaster(simpleNoticeBoard); Messenger simpleMessnger = new Messenger(simpleMessageBroadCaster); simpleMessnger.send(new String[] { "Welcome to Singleton" , "Town hall meet" }); } Note: We can avoid this if instead use Map, to look for specific type Creation of NoticeBoard becomes Abstract, using a program idiom

18. Module - IOC class MessengerModule{ private NoticeBoard noticeBoard; private MessageBroadCaster messageBroadCaster; private Messenger messenger; public MessengerModule(String type) { noticeBoard = new NoticeBoardFactory().createNoticeBoard(type); messageBroadCaster = new MessageBroadCaster(noticeBoard); messenger = new Messenger(messageBroadCaster); } public void send(String[] messages) { messenger.send(messages); } } MessengerModule simpleModule = new MessengerModule("Simple"); simpleModule.send(new String[] { "Welcome to Singleton" , "Town hall meet" }); MessengerModule lcdModule = new MessengerModule("LCD"); lcdModule.send(new String[] { "Welcome to Singleton" , "Town hall meet" }); MessengerModule electronicModule = new MessengerModule("Electronic"); electronicModule.send(new String[] { "Welcome to Singleton" , "Town hall meet" });

20. Flexible

21. Module – More Abstraction class MessengerModule{ private NoticeBoard noticeBoard; private MessageBroadCaster messageBroadCaster; private Messenger messenger; public MessengerModule(String type) { noticeBoard = new NoticeBoardFactory().createNoticeBoard(type); messageBroadCaster = new MessageBroadCasterFactory().createMessageBroadCaster(type,noticeBoard); messenger = new MessengerFactory().createMessenger(type, messageBroadCaster); } public void send(String[] messages) { messenger.send(messages); }}

23. class MessengerModule{ private NoticeBoard noticeBoard; private MessageBroadCaster messageBroadCaster; private Messenger messenger; private static final Map<String, MessengerModule> MSG_MODULES = new HashMap<String, MessengerModule>(); public static MessengerModule getInstance(String type) { MessengerModule messengerModule = MSG_MODULES.get(type); if(messengerModule == null) { messengerModule = new MessengerModule(type); MSG_MODULES.put(type,messengerModule); } return messengerModule; } private MessengerModule(String type) { noticeBoard = new NoticeBoardFactory().createNoticeBoard(type); messageBroadCaster = new MessageBroadCasterFactory().createMessageBroadCaster(type,noticeBoard); messenger = new MessengerFactory().createMessenger(type, messageBroadCaster);} public void send(String[] messages) { messenger.send(messages); } } Module – Singleton Classic

24. Module – Singleton – Java5 enum MessengerModule { Simple,LCD, Electronic; private NoticeBoard noticeBoard; private MessageBroadCaster messageBroadCaster; private Messenger messenger; public static MessengerModule getInstance(String type) { return valueOf(type); } private MessengerModule() { noticeBoard = new NoticeBoardFactory().createNoticeBoard(this.name()); messageBroadCaster = new MessageBroadCasterFactory().createMessageBroadCaster(this.name(),noticeBoard); messenger = new MessengerFactory().createMessenger(this.name(), messageBroadCaster); } public void send(String[] messages){ messenger.send(messages); }}

25. Test public class MessengerClient { public static void main(String[] args){ String[] messages = new String[] { &quot;Welcome to Singleton&quot; , &quot;Town hall meet&quot; }; MessengerModule.LCD.send(messages); MessengerModule.getInstance(&quot;Simple&quot;).send(messages); MessengerModule.getInstance(&quot;Electronic&quot;).send(messages); } } Output LCD Display ... Welcome to Singleton Town hall meet Simple Display ... Welcome to Singleton Town hall meet Electronic Display ... Welcome to Singleton Town hall meet

28. Thanks ...