Client.java

public class Client implements Observer{

	private String name;
	private String pw;

	public Client(String user,String pw) {
		this.name=user;
		this.pw=pw;
	}

	@Override
	public void update(Observable arg0, Object arg1) {
		// TODO Auto-generated method stub
		System.out.println ("Server -> Client "+name+arg1);
	}


	@Override
	public String toString() {
		// TODO Auto-generated method stub
		return "client "+name;
	}

	public String getName() {
		// TODO Auto-generated method stub
		return name;
	}

	public String getPW() {
		// TODO Auto-generated method stub
		return pw;
	}

	
	
}

Main.java

public class Main {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Client c1 = new Client("Thomas","mmbbs");
		Client c2= new Client("Frank","mbs");
		Client c3= new Client("Simone","mmbbs");
		Server s = new Server();
		
		s.addObserver(c1);
		s.addObserver(c2);		
		s.addObserver(c3);
		
		s.sendChat(new Message("","Nun sind alle da"));
		
		s.deleteObserver(c1);
		s.sendChat(new Message("","Nun ist einer weg"));
	}

}

Message.java

public class Message {

	private String from;
	private String msg;

	public  Message(String from,String msg) {
		this.from=from;
		this.msg=msg;
	}

	@Override
	public String toString() {
		// TODO Auto-generated method stub
		return from+":'"+msg+"'";
	}
	
	
}

Server.java

public class Server extends Observable{
	public void sendChat(Message m) {
		// TODO Auto-generated method stub
		setChanged();
		notifyObservers(m);
	}

	public synchronized void addObserver(Client c) {
		// TODO Auto-generated method stub
		if (c.getPW().compareTo("mmbbs")==0) {
			super.addObserver(c);
			this.sendChat(new Message("",c.getName()+" has joinded"));
		}
		else {
			c.update(this, new Message("","Login failed!"));
		}
	}

	public synchronized void deleteObserver(Client o) {
		// TODO Auto-generated method stub
		super.deleteObserver(o);
		this.sendChat(new Message("",o.getName()+" diconnected"));
	}
	
}