You may want to consider starting a new thread in the servlets forum as this one attracts mostly people interested in configuring and debugging Tomcat..
Okay PrintWriter I know. that's the class that lets me write the servlet's output to the HttpServletResponse. Why does it have a problem if the stream is already initialized? Picture a servlet trying to return 2 streams OR picture two pieces of code trying to write to the same stream. That codes insures that there is only one and that it's clean.