Problems with applet-servlet communication

public class HelloServlet extends HttpServlet
{

/* only for test purposes !
public void doGet(HttpServletRequest request, HttpServletResponse response)
{
try
{
FileWriter fw = new FileWriter(new File(“test.txt”));
fw.write(“doGet”);
fw.close();
}
catch (IOException e)
{

        }
  }

*/

  public void doPost(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException 
  {
        try {
              response.setContentType("application/x-java-serialized-object");

              InputStream in = request.getInputStream();
              ObjectInputStream inputFromApplet = new ObjectInputStream(in);
              String echo = (String) inputFromApplet.readObject();
              
              FileWriter fw = new FileWriter(new File("test.txt"));
              fw.write(echo);

              fw.close();


        } catch (Exception e) {
              e.printStackTrace();
        }
  }

}

now the applet, which submits a string:

public class HelloApplet extends Applet
{

  public void init() 
  {
        onSendData();
  }

  /**
   * Get a connection to the servlet.
   */
  private URLConnection getServletConnection()
        throws MalformedURLException, IOException {

        // Connection zum Servlet ýffnen
        URL urlServlet = new URL(getCodeBase(), "hello");
        URLConnection con = urlServlet.openConnection();

        // konfigurieren
        con.setDoInput(true);
        con.setDoOutput(true);
        con.setUseCaches(false);
        con.setRequestProperty(
              "Content-Type",
              "application/x-java-serialized-object");

        // und zurýckliefern
        return con;
  }

  private void onSendData() {
        try {
              // get input data for sending
              String input = "User 1            1:30";

              // send data to the servlet
              URLConnection con = getServletConnection();
              OutputStream outstream = con.getOutputStream();
              ObjectOutputStream oos = new ObjectOutputStream(outstream);
              oos.writeObject(input);
              oos.flush();
              oos.close();

              

        } catch (Exception ex) {
              ex.printStackTrace();
        }
  }

}

now i have several problems and questions.
first i get an exception:

java.net.UnknownServiceException: protocol doesn’t support output
at java.net.URLConnection.getOutputStream(Unknown Source)
at HelloApplet.onSendData(HelloApplet.java:54)
at HelloApplet.init(HelloApplet.java:19)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

grrrr, very sorry, posted too early, the intro is missing :-/

my applet needs to write a file onto the server using a servlet

therefore i have written two classes (with a little help of i-net articles) -->

first the server file :