一道JSP servlet的问题，大家看一下，问题在哪

我用servlet写了一个小页面，功能是计算两个随机数，若正确，显示下一组，若错误，继续做这题，下面是我写的代码
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;
import javax.servlet.http.*;/**
 *
 * @author VincentLee
 */
public class Sum extends HttpServlet {
   
    
  
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        HttpSession session=request.getSession(true);
        Integer cor=(Integer)session.getAttribute("correct");
        int num1;
        int num2;
        int c=1;
        if(cor==(Integer)c)
        { 
            num1=(Integer)session.getAttribute("num1");
           num2=(Integer)session.getAttribute("num2");
              
        }
        else
        {
          num1=(int)(100*Math.random());
 num2=(int)(100*Math.random());           
        }        session.setAttribute("num1",num1);
        session.setAttribute("num2", num2);
        System.out.println(session.getAttribute("num1"));
        
        try {
            
           response.setContentType("text/html"); out
.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
out.println("<HTML>");
out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");

out.println("  <BODY>");
              out.println(session.getAttribute("correct"));
                    out.println("num1="+session.getAttribute("num1"));
                    out.println("num2="+session.getAttribute("num2"));
out.println("<form>");
out.println("<table>");
out.println("<tr>");
out.println("<td>");
out.println("第一个数");
out.println("</td>");
out.println("<td>");
                out.println("<input type='text' name='num1' value="+num1+">");
out.println("</td>");
out.println("</tr>");
out.println("<tr>");
out.println("<td>");
out.println("第二个数");
out.println("</td>");
out.println("<td>");
                out.println("<input type='text' name='num2' value="+num2+">");
                out.println("</td>");
out.println("</tr>");
                out.println("<tr>");
out.println("<td>");
out.println("请输入结果");
out.println("</td>");
out.println("<td>");
out.println("<input type='text' name='Sum' value=''>");
out.println("</td>");
out.println("</tr>");
                out.println("<tr>");
out.println("<td>");
out.println("");
out.println("</td>");
out.println("<td>");
                out.println("<input type='Submit' name='sub' value='确定'>");
                out.println("</td>");
out.println("</tr>");
out.println("</table>");
out.println("</form>");
              
                  
                String Sum=request.getParameter("Sum").toString();
                int Sum1=Integer.parseInt(Sum);
                num1=Integer.parseInt(request.getParameter("num1").toString());
                 num2=Integer.parseInt(request.getParameter("num2").toString());
                 out.println("cor="+session.getAttribute("correct"));
               out.println("num1="+session.getAttribute("num1"));
                    out.println("num2="+session.getAttribute("num2"));
                 int Sum2=num1+num2;
               
                if(Sum1==Sum2)
                {
                    out.println("正确");
                      Integer x=0;
                    session.setAttribute("correct",x);
                }
                else
                {
                    out.println("错误");
                    Integer x=1;
                    session.setAttribute("correct",x);
                    out.println(session.getAttribute("correct"));
                    out.println("num1="+session.getAttribute("num1"));
                    out.println("num2="+session.getAttribute("num2"));
                }
               
out.println("  </BODY>");
out.println("</HTML>");
        }
        finally {            out.close();
        }
    }     
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        processRequest(request, response);
    }    
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        processRequest(request, response);
    } 
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>}
我出现的问题是，第一次错误后，它就再产生下一组随机数，然后再错的话就不会变了，我大概知道是表单提交与页面刷新顺序的原因，求大家说一下，应该怎么改，谢谢！！