编写一个函数将一个十六进制数的字符串参数转换成整数返回。

public static void main(String[] args) {
    System.out.println(getDecimalNum("85abf"));
}

public static long getDecimalNum(String hexStr) {
    int len = hexStr.length();
    long sum = 0;
    for (int i = 0; i < len; i++) {
        char c = hexStr.charAt(len - 1 - i);
        int n = Character.digit(c, 16);
        sum += n * (1 << (4 * i));
    }
    return sum;
}