Java校验银行卡是否正确的核心代码

/** 
   * 校验银行卡卡号 
   * 
   * @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; 
  } 
  /** 
   * 从不含校验位的银行卡卡号采用 Luhm 校验算法获得校验位 
   * 
   * @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("不是银行卡的卡号!"); 
    } 
    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'); 
  }