question: private Set mergeSet ( Set < String > set1 , Set < String > set2 ) { if ( set1 == null || set1 . isEmpty ( ) ) { if ( set2 == null || set2 . isEmpty ( ) ) { return Collections . EMPTY_SET ; } else { return set2 ; } } else { if ( set2 == null || set2 . isEmpty ( ) ) { return set1 ; } else { Set < String > returnSet = new HashSet < String > ( set1 ) ; returnSet . addAll ( set2 ) ; return returnSet ; } } }
target:  Return set which contains union of the two sets if one of the set is null or empty, return the other set