String newUsername = userDTO.getUsername();
if (oldUsername == null || newUsername == null || "".equals(newUsername)) {
throw new IllegalArgumentException("oldUsername and newUsername cannot be null or \"\" ");
}
if (newUsername.equals(oldUsername)) {
throw new UserAlreadyExistsException(newUsername);
}
if (getUser(newUsername) != null) {
throw new UserAlreadyExistsException(newUsername);
}
User user = getUser(oldUsername);
if (user == null) {
throw new UserNotFoundException(oldUsername);
}