code: public List<String> getUnknownWords() {
      if (!listUnknownWords) {
        throw new IllegalStateException("listUnknownWords is set to false, unknown words not stored");
      }
      List<String> words = new ArrayList<>(unknownWords);
      Collections.sort(words);
      return words;
    }
docs: Get the alphabetically sorted list of unknown words in the latest run of one of the {@link #check(String)} methods.
  @throws IllegalStateException if {@link #setListUnknownWords(boolean)} has been set to {@code false}