code: public static boolean isDiscover(String cc) {
          String first4digs = cc.substring(0, 4);
  
          if ((cc.length() == 16) && (first4digs.equals("6011")))
              return isCreditCard(cc);
          return false;
      }
docs: Checks to see if the cc number is a valid Discover number
  @param    cc - a string representing a credit card number; Sample number: 6011000000000004(16 digits)
  @return  true, if the credit card number is a valid Discover card number, false otherwise