private void registerUser(String user, String password) throws RegistrationException{
   	 if(user == null || password == null){
   		 throw new RegistrationException("Username or password was inValid.");
   	 }
   	 if(user_exists(user)){
   		 throw new RegistrationException("Username already exist.");
   	 }
   	 if(password.length() < 6){
   		 throw new RegistrationException("Password must be at least 6 characters long.");
   	 }
   	 
   	 try {
   		 save_accounts(user, password);
   		 accounts.put(user, password);
   	 } catch (IOException e) {
   		 throw new RegistrationException("Server was not able to save your account.");
   	 }
    
   	 
   	 
   	 return;
 
    }