code: public double tfidfWord(String word, long wordCount, long documentLength) {
          double tf = tfForWord(wordCount, documentLength);
          double idf = idfForWord(word);
          return MathUtils.tfidf(tf, idf);
      }
docs: Calculate the tifdf for a word
  given the word, word count, and document length
  @param word the word to calculate
  @param wordCount the word frequency
  @param documentLength the number of words in the document
  @return the tfidf weight for a given word