public static void main(String[] args) {
        System.out.println(getEmailSuffix("jim_chen28270@163.com")); // 163.com
        System.out.println(getEmailSuffix("@qq.com")); //qq.com
    }

    private static String getEmailSuffix(String email) {
        email = defaultIfBlank(email,"").trim();
        if (!email.isEmpty()) {
            int index = email.indexOf(‘@‘) + 1;
            if (index < email.length()) {
                return email.substring(index).toLowerCase();
            }
        }
        return "";
    }

    public static <T extends CharSequence> T defaultIfBlank(final T str, final T defaultStr) {
        return isBlank(str) ? defaultStr : str;
    }

    public static boolean isBlank(final CharSequence cs) {
        int strLen;
        if (cs == null || (strLen = cs.length()) == 0) {
            return true;
        }
        for (int i = 0; i < strLen; i++) {
            if (!Character.isWhitespace(cs.charAt(i))) {
                return false;
            }
        }
        return true;
    }
}

 public static String getEmailSuffix(String email) {
        email = MyStringUtils.defaultIfBlank(email,"").trim();
        if (!email.isEmpty()) {
            int index = email.indexOf(‘@‘) + 1;
            if (index < email.length()) {
                return email.substring(index).toLowerCase();
            }
        }
        return "";
    }

//自己封装的字符串操作类
class MyStringUtils {
    public static <T extends CharSequence> T defaultIfBlank(final T str, final T defaultStr) {
        return isBlank(str) ? defaultStr : str;
    }

    public static boolean isBlank(final CharSequence cs) {
        int strLen;
        if (cs == null || (strLen = cs.length()) == 0) {
            return true;
        }
        for (int i = 0; i < strLen; i++) {
            if (!Character.isWhitespace(cs.charAt(i))) {
                return false;
            }
        }
        return true;
    }

    public static boolean isNotBlank(final CharSequence cs) {
        return !isBlank(cs);
    }

    public static String defaultString(Object obj) {
        return obj == null ? "" : obj.toString();
    }
}