private static void padWithNulls(final Collection<String> s1, final Collection<String> s2) {
    if (s1.size() != s2.size()) {
      while (s1.size() < s2.size()) {
        s1.add(null);
      }
      while (s1.size() > s2.size()) {
        s2.add(null);
      }
    }
  }
private static void padWithNulls(final Collection<String> s1, final Collection<String> s2)