private UserDTO getUser(Long id) {
   if (user == null || !user.getId().equals(id)) {
     user = userService.getUserById(id);
   }
   return user;
 }
 public void visitUsers(UserVisitor visitor) {
   for (UserDTO u : userService.getAllUsers()) {
     visitor.visit(u.getId(), u.getLastName(), u.getFirstName());
   }
 }