code: public String getClassIdentity(XEvent event) {
  		StringBuilder sb = new StringBuilder();
  		for (int i = 0; i < keys.length; i++) {
  			XAttribute attribute = event.getAttributes().get(keys[i]);
  //			if (attribute == null) {
  //				return null;
  //			}
  			if (attribute != null) {
  				sb.append(attribute.toString().trim());
  			}
  			if (i < (keys.length - 1)) {
  				sb.append("+");
  			}
  		}
  		return sb.toString();
  	}
docs: /*
  (non-Javadoc)
  
  @see
  org.deckfour.xes.classification.XEventClassifier#getClassIdentity(org
  .deckfour.xes.model.XEvent)