code: public void addFeature(String feature) {
          if (feature == null) {
              throw new IllegalArgumentException("Invalid null argument passed for addFeature");
          }
          feature = feature.trim();
          if (!feature.isEmpty()) { 
              Feature newFeature = new Feature();
              newFeature.addText(feature);
              featureList.add(newFeature);
          }
      }
docs: Add a feature into a list.
  
  @param feature A feature name.