@Override
    public String toString() {
        if (persons != null && persons.size() == 1) {
            return persons.get(0).getValue();
        } else if (emails != null && emails.size() == 1) {
            return emails.get(0).getValue();
        } else if (organisations != null && organisations.size() == 1) {
            return organisations.get(0).getValue();
        } else if (urls != null && urls.size() == 1) {
            return urls.get(0).toString();
        } else {
            return null;
        }
    }