RandomStringUtils generate similar values in one run. How to generate different?

I have a problem. In one test run i generate random numeric variable (but has String type) in every test. Im using a RandomStringUtils lib for that.

But it generates only one value of that variable for all of tests. How to generate new value in every test?

Example:

public void test() {
    JsonResponse response = send(baseUrl(), generateId());
    response.status200();
}

and generateId() method in other class

public static String generateId(){
        return RandomStringUtils.randomNumeric(6);
    }

thanks.