/**
 * 检查注册码
 * @return
 */
public Boolean checkSN() {
    String username = et_username.getText().toString();
    String password = et_password.getText().toString();
    if (username.length() == 0) {
        return false;
    }
    if (password.length() != 16) {
        return false;
    }
    try {
        MessageDigest messageDigest = MessageDigest.getInstance("MD5");
        messageDigest.digest();
        messageDigest.update(username.getBytes());
        byte[] bytes = messageDigest.digest();
        String hexstr = toHexString(bytes, "");
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < hexstr.length(); i += 2){
            stringBuilder.append(hexstr.charAt(i));
        }
        String userSN = stringBuilder.toString();
        if (!userSN.equalsIgnoreCase(password)) {
            return false;
        }

    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
        return false;
    }
    return true;
}