code: public static String getFieldsFor(Class<?> dataClass) {
      StringBuilder fieldsBuf = new StringBuilder();
      appendFieldsFor(fieldsBuf, dataClass, new int[1]);
      return fieldsBuf.toString();
    }
docs: Returns the fields mask to use for the given data class of key/value pairs. It cannot be a
  {@link Map}, {@link GenericData} or a {@link Collection}.
  
  @param dataClass data class of key/value pairs