public List<Feature> extract(JCas aJcas, TextClassificationUnit aClassificationUnit)
      throws TextClassificationException {
    init();

    String unit = aClassificationUnit.getCoveredText();

    unit = substituteHashtags(unit);
    unit = substituteUrl(unit);
    unit = substituteAtMentions(unit);

    List<Feature> features = createFeatures(unit);

    return features;
  }