More talk, less code:

/**
   *  Verify bank card number
   *
   * @param cardId
   * @return
   */
  public static boolean checkBankCard(String cardId) {
    char bit = getBankCardCheckCode(cardId
        .substring(0, cardId.length() - 1));
    return cardId.charAt(cardId.length() - 1) == bit;
  }
  /**
   *  Bank card Numbers that do not contain checksum bits are used  Luhm  The calibration algorithm obtains the calibration bit
   *
   * @param nonCheckCodeCardId
   * @return
   */
  public static char getBankCardCheckCode(String nonCheckCodeCardId) {
    int cardLenth = nonCheckCodeCardId.trim().length();
    if (nonCheckCodeCardId == null || cardLenth == 0
        || !nonCheckCodeCardId.matches("\\d+")) {
      throw new IllegalArgumentException(" It's not the card number !");
    }
    char[] chs = nonCheckCodeCardId.trim().toCharArray();
    int luhmSum = 0;
    for (int i = chs.length - 1, j = 0; i >= 0; i--, j++) {
      int k = chs[i] - '0';
      if (j % 2 == 0) {
        k *= 2;
        k = k / 10 + k % 10;
      }
      luhmSum += k;
    }
    return (luhmSum % 10 == 0) ? '0' : (char) ((10 - luhmSum % 10) + '0');
  }

Feel this copy past can… So I didn’t go into it. It felt like a waste of time.