question: private static Map < String , Set < String > > parseSparseFields ( MultivaluedMap < String , String > queryParams ) { Map < String , Set < String > > result = new HashMap < > ( ) ; for ( Map . Entry < String , List < String > > kv : queryParams . entrySet ( ) ) { String key = kv . getKey ( ) ; if ( key . startsWith ( "fields[" ) && key . endsWith ( "]" ) ) { String type = key . substring ( 7 , key . length ( ) - 1 ) ; LinkedHashSet < String > filters = new LinkedHashSet < > ( ) ; for ( String filterParams : kv . getValue ( ) ) { Collections . addAll ( filters , filterParams . split ( "," ) ) ; } if ( ! filters . isEmpty ( ) ) { result . put ( type , filters ) ; } } } return result ; }
target:  Parses queryParams and produces sparseFields map.