public boolean equals(Object other) {
        if (other == this)
            return true;
        if (! (other instanceof Citizen) )
            return false;
        Citizen otherCitizen = (Citizen) other;
        return (fullName == null) ?
                (otherCitizen.fullName == null) :
        (fullName.equals(otherCitizen.fullName) && officialId.equals(otherCitizen.officialId));
    }