ajax problem - 200 OK in firebug but red message with no response body

You need to use a domain-relative URL in your Ajax request:

/Dinamicki1/UsernameServlet?username=zik

Or a context-relative URL (assuming that the page is served from /Dinamicki1):

UsernameServlet?username=zik

With regard to "Ajax tutorial for Java", start here: How to use Servlets and Ajax?

You cannot use AJAX to read replies from other domains.

Your HTML must be on the same server (and same domain, port, and protocol) as the AJAX servlet.