code: public static <F, S> Pair<F, S> of(F first, S second) {
          if (first == null || second == null) {
              throw new IllegalArgumentException("Pair.of requires non null values.");
          }
          return new Pair<F, S>(first, second);
      }
docs: Creates a pair object
  @param first must be non-null
  @param second must be non-null
  @return The pair object.