public String display()
    {
        String str ;
        switch( num_digits )
        {
            case 0: str = "(_ _ _) _ _ _ - _ _ _ _" ; break ;
            case 1: str = "(123) _ _ _ - _ _ _ _" ; break ;
            case 2: str = "(123) 456 - _ _ _ _" ; break ;
            case 3:  str = "(123) 456 - 7890 " ;  break ;
            default: str = "_ _ _ _ _ _ _ _ _ _" ;
        }
        System.out.println( str ) ;
        return str ;    
    }