question: public static String withFirst ( String input , Function < Character , String > callback ) { if ( input == null ) { return null ; } else if ( input . length ( ) == 0 ) { return EMPTY ; } else { return String . join ( EMPTY , callback . apply ( input . charAt ( 0 ) ) , input . subSequence ( 1 , input . length ( ) ) ) ; } }
target:  Does something with the first character in the specified String.