@Override
  public int hashCode() {
    if (first == null) return (second == null) ? 0 : second.hashCode() + 1;
    else if (second == null) return first.hashCode() + 2;
    else return first.hashCode() * 17 + second.hashCode();
  }
Return the second element stored in the pair.
hbase-common/src/main/java/org/apache/hadoop/hbase/util/PairOfSameType.java