VariantA.java

public UserService {
  
public User register(
      @NonNull final String username,
      @NonNull final String plainPassword,
      @NonNull final Email email,
      @NonNull final LocalDate birthDate,
      @NonNull final Gender gender) throws UnableToSaveUserException {}
  
}

// multiple exception can occurs as source of UnableToSaveUserException
public class UnableToSaveUserException extends Exception {}

public class UsernameAlreadyRegisteredException extends UnableToSaveUserException {}

public class EmailAlreadyRegisteredException extends UnableToSaveUserException {}

public class NotWellFormedUserDtoException extends UnableToSaveUserException {}

public class UserNotAuthorizedToPerformOperation extends RuntimeException {} // should it be a checked exception ????