docs: public static <BEAN, KEYTYPE> Set<KEYTYPE> getNoDataKeySet(Map<KEYTYPE, BEAN> map) {
  		Set<KEYTYPE> keySet = null;
  		Iterator<Entry<KEYTYPE, BEAN>> iterator = map.entrySet().iterator();
  		while (iterator.hasNext()) {
  			Entry<KEYTYPE, BEAN> entry = iterator.next();
  			BEAN bean = entry.getValue();
  			if (bean == null) {
  				if (keySet == null) {
  					keySet = new LinkedHashSet<KEYTYPE>();
  				}
  				keySet.add(entry.getKey());
  			}
  		}
  		return keySet;
  	}