question: public static StringBuilder formatTo ( StringBuilder buf , double [ ] d , String sep , NumberFormat nf ) { if ( d == null ) { return buf . append ( "null" ) ; } if ( d . length == 0 ) { return buf ; } buf . append ( nf . format ( d [ 0 ] ) ) ; for ( int i = 1 ; i < d . length ; i ++ ) { buf . append ( sep ) ; buf . append ( nf . format ( d [ i ] ) ) ; } return buf ; }
target:  Formats the double array d with the specified number format.