checkNotNull(userName);
checkNotNull(email);
checkNotNull(password);
User existingUser = metaproject.getUser(userName);
if (existingUser != null) {
throw new UserNameAlreadyExistsException(userName);
}
for(User user : metaproject.getUsers()) {
if(email.equals(user.getEmail())) {
throw new UserEmailAlreadyExistsException(email);
}