信用卡校驗Java版

看了http://demo.tc/view.aspx?id=398這篇文章有感而發寫出Java版本, 以後應該有機會用到。

信用卡校驗規則為:

1.必須是十六個數字.
2.由左至右取出奇數位數字乘2, 如果大於9則減去9.
3.合併奇數位與偶數位每一個數字總合必須能夠被10整除.
4.開頭第一個字元為4則是Visa, 頭兩個字元大於50則為Master.

注意一下Java char轉int 不可以用(int)char強制轉, 否則會變成ascii code, 請使用Character的digit(char ch, int radix).我們要轉十進位數字所以radix填10.

The Character class wraps a value of the primitive type char in an object.
digit(char ch, int radix) Returns the numeric value of the character ch in the specified radix.

Sample Code:

public static String checkCreditCard(String cardNo)
{
    String result = "Wrong card number!!";
    try {
        // length validation must bigger than sixteen.
        int length = cardNo.length();
        if (length == 16) {

            String type = "";
            if (cardNo.substring(0, 2).compareTo("50") >= 0) {
                type = "Master";
            } else if (cardNo.substring(0, 1).equals("4") == true) {
                type = "Visa";
            }

            // separate the card number.
            char[] sepCard = cardNo.toCharArray();
            // create a new array.
            int[] newCard = new int[16];

            for (int i = 0; i < sepCard.length; i++) {
                if ((i + 1) % 2 == 0) {
                    newCard[i] = Character.digit(sepCard[i], 10);
                } else {
                    newCard[i] = Character.digit(sepCard[i], 10) * 2;
                    if (newCard[i] > 9)
                        newCard[i] -= 9;
                }
            }

            int sum = 0;
            for (int i = 0; i < newCard.length; i++) {
                sum += newCard[i];
            }

            if (sum % 10 == 0)
                result = type + " card.";
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    return result;
}