public TrackedEntityInstanceQueryParams addAttributesIfNotExist(List<QueryItem> attrs){
  for (  QueryItem attr : attrs) {
    if (attributes != null && !attributes.contains(attr)) {
      attributes.add(attr);
    }
  }
  return this;
}

public TrackedEntityInstanceQueryParams addAttributesIfNotExist ( List < QueryItem > attrs ) { for ( QueryItem attr : attrs ) { if ( attributes != null && ! attributes . contains ( attr ) ) { attributes . add ( attr ) ; } } return this ; }
add the given attributes to this params if they are not already present .
