help for shopping cart

dear ones
i am doing project in online shopping.my project contains store for books,cars,watches etc.my problem is how to develop a shopping cart which shows deatils about items selected when user clicks addto bag
here is my sample html page which is a bookstore
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body bgcolor="#FFFFCC" text="#000000">
1. Java Servlets
<pre align="left"> Author :Karl Moss
Price :275.00
Publisher :
ID :12457 <a href="/servlet/showbag?bookid=12457&bname=java servlets&price=275.00" target="_blank" >Add to Bag</a></pre>
<hr>
2.Java Server Pages
<div align="left">
<pre>Author :
Price :
Publisher :
ID : 12359 <a href="/servlet/showbag?bookid=12359&bname=Java Server Pages&price=456.00" target="_blank">Add to Bag</a></pre>
<hr>
<h3>3.Programming in C</h3>
<pre>Author :Balaguruswamy
Price :150.00
Publisher :
ID :12358 <a href="/servlet/bk1?bookid=12358" target="_blank">Add to Bag</a></pre>
<hr>
4.Visual C++
<pre>Author :
Price :
Publisher :
ID :12369 <a href="/servlet/bk1?bookid=12369" target="_blank">Add to bag</a></pre>
<hr>
<h2>5.Computer Networks</h2>
<pre>Author :Tannenbaum
Price :200
Publisher:
ID : 12457 <a href="/servlet/bk1?bookid=12459" target="_blank">Add to bag</a></pre>
<hr>
<pre> <a href="page3.htm" target="_blank"><img src="home.jpg" width="160" height="34" border="0"></a></pre>
</div>
</body>
</html>
ihave written one servlet for shopping cart but shows error500
the servler is
package onlineshop;
// import online.BooksDetails;
import java.util.*;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ShowCartServlet extends HttpServlet
{
public BooksDetails cart;
public void doGet (HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{

response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("ppp");
HttpSession session=request.getSession(true);
cart=(BooksDetails)session.getValue(session.getId());

if (cart == null)
{
cart = new BooksDetails();
session.putValue(session.getId(), cart);
}

addCart(request);
showCart(out);
}
private void addCart(HttpServletRequest request)
{
int units=1;
String bookid=request.getParameter("bookid");
double price=Double.valueOf(request.getParameter("price")).doubleValue();
String bname =request.getParameter("bname");
cart.additem(bname,bookid,units,price);
}
private void showCart(PrintWriter out)
{
//out.println("<center> Your user ID: "+cart.cid+" ");
// show empty info if there's nothing in shopping cart
if (cart.totalItems==0){
out.println("<h3>Sorry, there's nothing in your shopping cart yet!</h3>");}
else {
out.println("<center><table border=4 background=\"/bg.gif\" width = 90% >");
out.println("<tr>");
out.println("<td><center>Bookname</center></td>");
out.println("<td><center>Bookid</center></td>");
out.println("<td><center>Price</center></td>");
out.println("<td><center>Quantity</center></td>");
out.println("<td><center>SubAmount</center></td>");
out.println("</tr>");
for (int i = 0; i < cart.totalItems; i++){
out.println("<tr>");
out.println("<td><center> "+cart.bname[i]+" </center></td>");
out.println("<td><center> "+cart.bookid[i]+" </center></td>");
out.println("<td><center> "+cart.unitprice[i]+" </center></td>");
out.println("<td><center> $"+cart.units[i]+" </center></td>");
out.println("<td><center> $"+cart.stotal[i]+" </center></td>");
out.println("</tr>");
}
out.println("</table>");
}
}

// package online;
// import java.io.*;
// import java.util.*;
class BooksDetails
{
public String[] bname;
public String[] author;
public String[] bookid;
public int[] units;
public double[] unitprice;
public double[] stotal;
public double totalPrice =0;
public int cid=0;
public String password="";
public int maxItems = 100;
public int totalItems =0;

public void BooksDetails()
{
bname=new String[maxItems];
author=new String[maxItems];
units=new int[maxItems];
bookid=new String[maxItems];
stotal=new double[maxItems];
}
public BooksDetails additem(String bname,String bookid,int units,double unitprice)
{
if(!found(bookid))
{
this.bname[totalItems]=bname;
this.bookid[totalItems]=bookid;
this.units[totalItems]=units;
this.unitprice[totalItems]=unitprice;
this.stotal[totalItems]=units*unitprice;
totalItems++;
}
totalPrice=getPrice();
return this;
}
public boolean found(String bookid)
{
for(int i=0;i<totalItems;i++)
{
if (this.bookid[i].equals(bookid))
{
this.units[i]++;
this.stotal[i]=this.units[i]*this.unitprice[i];
return true;
}
}
return false;
}
public String[] showCart() {
String[] s = new String[totalItems];
for (int i = 0; i < totalItems; i++)
{
s[i] = "<td><center> "+bname[i]+" </center></td>";
s[i] += "<td><center> "+bookid[i]+" </center></td>";
s[i] += "<td><center> $"+unitprice[i]+" </center></td>";
s[i] += "<td><center><input type=\"text\" size=\"5\" name=\"items\"> "+units[i]+" </center></td>";
s[i] += "<td><center> $;"+stotal[i]+" </center></td>";
}
return s;
}
public double getPrice() {
double p = 0.0;
for (int i = 0; i < totalItems; i++){
p += units[i]*unitprice[i];
}
return p;
}
}
please help m with correction if possible or a new source code for the shopping cart