@Override
 public void incrementCount(String word, double by) {
   wordFrequencies.incrementCount(word, by);
   if (wordFrequencies.getCount(word) >= minWordFrequency && vocabWords.indexOf(word) < 0)
     vocabWords.add(word);
 }
 @Override
 public String wordAt(int i) {
   return vocabWords.get(i).toString();
 }