Servlet question

Hi,
I have a question about an example of servlet that illustrates the use of session.

This servlet forwards the user first to the url "urlPage0" where the user needs to introduce a name.
So urlPage0 is a JSP with a form, with textfield asking for a name.
The value of the action of the form is the servlet and it gives a hidden field "étape" with the value 1.
Then the servlet forwards the user to "urlPage1" where the name is printed and asks for an age.
A hidden parameter is given again "étape" with value 2.
Then finally the user gets forwarded to "urlPage2" where name and age are printed.

I don't understand the use of the line request.setAttribute("nom",""); in étape0 method
like request.setAttribute("age",""); in étape1.

and what is the use of
request.setAttribute("nom",nom);
followed by
session.setAttribute("nom",nom);



public class main extends HttpServlet{
String msgErreur=null;
String urlPage0=null;
String urlPage1=null;
String urlPage2=null;
String urlErreur=null;



//-------- INIT
public void init(){
      ServletConfig config=getServletConfig();
      urlPage0=config.getInitParameter("urlPage0");
      urlPage1=config.getInitParameter("urlPage1");
      urlPage2=config.getInitParameter("urlPage2");
      urlErreur=config.getInitParameter("urlErreur");
      if(urlPage0==null || urlPage1==null || urlPage2==null){
            msgErreur="Configuration incorrecte";
      }
}



//-------- GET
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException{
      if(msgErreur!=null){
            getServletContext().getRequestDispatcher(urlErreur).forward(request,response);
      }
      String étape=request.getParameter("etape");
      HttpSession session=request.getSession();
      if(étape==null) étape0(request,response,session);
      if(étape.equals("1")) étape1(request,response,session);
      if(étape.equals("2")) étape2(request,response,session);
      étape0(request,response,session);
}




//-------- POST
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException{
      doGet(request,response);
}




//-------- étape0
public void étape0(HttpServletRequest request, HttpServletResponse response, HttpSession session)
throws IOException, ServletException{
      request.setAttribute("nom","");
      request.getRequestDispatcher(urlPage0).forward(request,response);
}




//-------- étape1
public void étape1(HttpServletRequest request, HttpServletResponse response, HttpSession session)
throws IOException, ServletException{
      String nom=request.getParameter("nom");
      if(nom==null) étape0(request,response,session);
      nom=nom.trim();
      request.setAttribute("nom",nom);
      if(nom.equals("")){
            ArrayList erreurs=new ArrayList();
            erreurs.add("Nous n'avez pas indiqué de nom");
            request.setAttribute("erreurs",erreurs);
            étape0(request,response,session);
      }
      session.setAttribute("nom",nom);
      request.setAttribute("age","");
      request.getRequestDispatcher(urlPage1).forward(request,response);
}





//-------- étape2
public void étape2(HttpServletRequest request, HttpServletResponse response, HttpSession session)
throws IOException, ServletException{
      String nom=(String)session.getAttribute("nom");
      if(nom==null) étape0(request,response,session);
      request.setAttribute("nom",nom);
      String age=request.getParameter("age");
      if(age==null){
            request.setAttribute("age","");
            request.getRequestDispatcher(urlPage1).forward(request,response);
      }
      age=age.trim();
      request.setAttribute("age",age);
      if(! Pattern.matches("^\\s*\\d+\\s*$",age)){
            ArrayList erreurs=new ArrayList();
            erreurs.add("Age invalide");
            request.setAttribute("erreurs",erreurs);
            request.getRequestDispatcher(urlPage1).forward(request,response);
      }
      request.getRequestDispatcher(urlPage2).forward(request,response);
}
}