question: public static void appendColumns ( StringBuilder s , String [ ] columns ) { int n = columns . length ; for ( int i = 0 ; i < n ; i ++ ) { String column = columns [ i ] ; if ( column != null ) { if ( i > 0 ) { s . append ( ", " ) ; } s . append ( column ) ; } } s . append ( ' ' ) ; }
target:  Add the names that are non-null in columns to s, separating them with commas.