code: public static PairFrequency create(String first, char delimiter ,String second)
      {
          PairFrequency pairFrequency = new PairFrequency(first + delimiter + second);
          pairFrequency.first = first;
          pairFrequency.delimiter = delimiter;
          pairFrequency.second = second;
          return pairFrequency;
      }
docs: 构造一个pf
  @param first
  @param delimiter
  @param second
  @return