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();

    }

}