Testing my java servlet with Junit?

I have created a simple calculator using java servlets, and I have been told to test it with JUNIT.

How do I test servlets with http responses and such, could some one give me an example of a test?

This is an example of my code, parameters are submitted by a form in html;

package NipenderSoft;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LaskuriServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("text/html;charset=UTF-8");
    try{
        laske(request, response);
    }catch (Exception e){
        System.out.println("fail");
    }
    PrintWriter temp = response.getWriter();
}

 private void laske(HttpServletRequest request, HttpServletResponse response) {
    try
    {
    response.setContentType("text/html");
    PrintWriter out= response.getWriter();
    int luku01= Integer.parseInt(request.getParameter("luku01"));
    int luku02= Integer.parseInt(request.getParameter("luku02"));
    if(request.getParameter("plus")!=null)
    {
        out.println("<h1>Plussaaminen</h1>"+(luku01+luku02));
    }
    if(request.getParameter("miinus")!=null)
    {
        out.println("<h1>Miinustaminen</h1>"+(luku01-luku02));
    }
    if(request.getParameter("kerto")!=null)
    {
        out.println("<h1>Kertolasku</h1>"+(luku01*luku02));
    }if(request.getParameter("jako")!=null)
    {
        out.println("<h1>Jakolasku</h1>"+(luku01/luku02));
    }
    }
    catch(Exception e)
    {

    }

}