Junit testcase for uniqueId method generator

Hi All,

Could you please let me know how to write junit for the below method generating randomId in sling model.

public String getRandomId() {

    int n=8;

    byte[] array = new byte[256];

    new Random().nextBytes(array);

 String randomString = new String(array, Charset.forName("UTF-8"));

    StringBuffer r = new StringBuffer();

    for (int k = 0; k < randomString.length(); k++) {

        char ch = randomString.charAt(k);

        if (((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') || (ch >= '0' && ch <= '9'))&& (n > 0)) {

            r.append(ch);

            n--;

        }

    }

    return "_"+r.toString();

}

Thanks in Advance!