code: public static String getFeedFields(Class<?> feedClass, Class<?> entryClass) {
      StringBuilder fieldsBuf = new StringBuilder();
      appendFeedFields(fieldsBuf, feedClass, entryClass);
      return fieldsBuf.toString();
    }
docs: Returns the fields mask to use for the given data class of key/value pairs for the feed class
  and for the entry class. This should only be used if the feed class does not contain the entry
  class as a field. The data classes cannot be a {@link Map}, {@link GenericData} or a
  {@link Collection}.
  
  @param feedClass feed data class
  @param entryClass entry data class