把十六进制字符转换成十进制数

    /**
     * Get the hex value of a character (base16).
     * @param c A character between '0' and '9' or between 'A' and 'F' or
     * between 'a' and 'f'.
     * @return  An int between 0 and 15, or -1 if c was not a hex digit.
     */
    public static int dehexchar(char c) {
        if (c >= '0' && c <= '9') {
            return c - '0';
        }
        if (c >= 'A' && c <= 'F') {
            return c - ('A' - 10);
        }
        if (c >= 'a' && c <= 'f') {
            return c - ('a' - 10);
        }
        return -1;
    }