SESSION TRACKING.

PLEASE HELP ME IN UNDERSTANDING COOKIES.ALSO HTTPSESSION INTERFACE FUNDAS.

Heck - there's a good example provided with Tomcat:
Just remember, cookies are nothing more than small files stored on the user's hard drive that have a certain expiration date. You can store up to 4k of string data in any cookie. If you encode your data properly, you can remember all sorts of things....userid, password, preferences, etc.
Always remember, the user can delete their cookies at any time or turn them off at the browser....therefore it is always good practice to check for this condition first.....
Tomcat and other apps use a single cookie to track the sessionid of the user and the cookie name is the same as the session id.

/* $Id: CookieExample.java,v 1.1.1.1 1999/10/09 00:19:59 duncan Exp $
*
*/
import java.io.*;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
/**
* Example servlet showing request headers
*
* @author James Duncan Davidson <duncan@eng.sun.com>
*/
public class CookieExample extends HttpServlet {
ResourceBundle rb = ResourceBundle.getBundle("LocalStrings");

public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<body bgcolor=\"white\">");
out.println("<head>");
String title = rb.getString("cookies.title");
out.println("<title>" + title + "</title>");
out.println("</head>");
out.println("<body>");
// relative links
// XXX
// making these absolute till we work out the
// addition of a PathInfo issue

out.println("<a href=\"/examples/servlets/cookies.html\">");
out.println("<img src=\"/examples/images/code.gif\" height=24 " +
"width=24 align=right border=0 alt=\"view code\"></a>");
out.println("<a href=\"/examples/servlets/index.html\">");
out.println("<img src=\"/examples/images/return.gif\" height=24 " +
"width=24 align=right border=0 alt=\"return\"></a>");
out.println("<h3>" + title + "</h3>");
Cookie[] cookies = request.getCookies();
if (cookies.length > 0) {
out.println(rb.getString("cookies.cookies") + "<br>");
for (int i = 0; i < cookies.length; i++) {
Cookie cookie = cookies[i];
out.print("Cookie Name: " + cookie.getName() + "<br>");
out.println(" Cookie Value: " + cookie.getValue() +
"<br><br>");
}
} else {
out.println(rb.getString("cookies.no-cookies"));
}
String cookieName = request.getParameter("cookiename");
String cookieValue = request.getParameter("cookievalue");
if (cookieName != null && cookieValue != null) {
Cookie cookie = new Cookie(cookieName, cookieValue);
response.addCookie(cookie);
out.println("<P>");
out.println(rb.getString("cookies.set") + "<br>");
out.print(rb.getString("cookies.name") + " " + cookieName +
"<br>");
out.print(rb.getString("cookies.value") + " " + cookieValue);
}

out.println("<P>");
out.println(rb.getString("cookies.make-cookie") + "<br>");
out.print("<form action=\"");
out.println("CookieExample\" method=POST>");
out.print(rb.getString("cookies.name") + " ");
out.println("<input type=text length=20 name=cookiename><br>");
out.print(rb.getString("cookies.value") + " ");
out.println("<input type=text length=20 name=cookievalue><br>");
out.println("<input type=submit></form>");

out.println("</body>");
out.println("</html>");
}
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
doGet(request, response);
}
}