求高手帮忙解决一下问题Java Web Cookie实例

package cn.com;
 
import java.io.IOException;
import java.io.PrintWriter;
import java.util.LinkedHashMap;
import java.util.Map;
 
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
public class CookieDemo3 extends HttpServlet {
    private static final long serialVersionUID = 1L;
 
     
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //解决中文乱码问题
        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out=response.getWriter();
         
        //输出所有的商品
        out.write("本网站有如下商品：<br/>");
        Map<String ,Book > map=Db.getAll();
        for(Map.Entry<String, Book> entry:map.entrySet())
        {
            Book book=entry.getValue();
            out.print("<a href='/Cookie/CookieDemo4?id=' "+book.getId()+" target='_blank'>"+book.getName()+"</a><br/>");
        }
         
         
        //显示用户曾经看过的商品
        Cookie cookies[]=request.getCookies();
        for(int i=0;cookies!=null&&i<cookies.length;i++)
        {
            if(cookies[i].getName().equals("bookHistory"))
            {
                String ids[]=cookies[i].getValue().split("\,");
                for(String id:ids)
                {
                    Book book=(Book)Db.getAll().get(id);
                    out.print(book.getName()+"<br/>");
                }
            }
        }
         
         
    }
 
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
     
        doGet(request,response);
    }
 
}
//Db作为数据库
class Db
{
    private static Map<String ,Book> map=new LinkedHashMap();
    static
    {
        map.put("1", new Book("1","Java WEB开发","WY","好书"));
        map.put("2", new Book("2","WEB开发","zt","一般"));
        map.put("3", new Book("3","程序设计","df","较好书"));
        map.put("4", new Book("4","计算机组成","as","一般好书"));
        map.put("5", new Book("5","编译原理","ty","很好书"));
        map.put("6", new Book("6","网络维护","hj","非常好书"));
    }
     
    public static Map getAll()
    {
        return map;
    }
}
//书
class Book
{
    private String id;
    private String name;
    private String author;
    private String description;
     
    public Book() {
        super();
        // TODO Auto-generated constructor stub
    }
    public Book(String id, String name, String author, String description) {
        super();
        this.id = id;
        this.name = name;
        this.author = author;
        this.description = description;
    }
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAuthor() {
        return author;
    }
    public void setAuthor(String author) {
        this.author = author;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
     
     
}
package cn.com;
 
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Iterator;
import java.util.LinkedList;
 
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
/**
 * Servlet implementation class CookieDemo4
 */
public class CookieDemo4 extends HttpServlet {
    private static final long serialVersionUID = 1L;
     
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         
        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out=response.getWriter();
         
        //根据用户带过来的id ,显示响应商品的详细信息
        String id=request.getParameter("id");
        Book book=(Book)Db.getAll().get(id);
         
        out.write(book.getId()+"<br/>");
        out.write(book.getAuthor()+"<br/>");
        out.write(book.getDescription()+"<br/>");
        out.write(book.getName()+"<br/>");
         
        //构建Cookie ,回写给浏览器
        String cookieValue=buildCookie(id,request);
        Cookie cookie=new Cookie("bookHistory",cookieValue);
        cookie.setMaxAge(30*24*3600);
        cookie.setPath("/Cookie");
        response.addCookie(cookie);
    }
 
     
    private String buildCookie(String id, HttpServletRequest request) {
        // TODO Auto-generated method stub
        //如果用户没有带任何的Cookie过来
         
        //如果用户带了Cookie来了包含在看过的id中，则要把id删掉
         
        //带了Cookie过来了但是没有包含看过的id
         
        //带了Cookie过来但是没有包含
         
        String bookHistory=null;
        Cookie cookies[]=request.getCookies();
        for(int i=0;cookies!=null&&i<cookies.length;i++)
        {
            if(cookies[i].getName().equals("bookHistory"))
            {
                bookHistory=cookies[i].getValue();
            }
        }
        if(bookHistory==null)
        {
            return id;
        }
        @SuppressWarnings("rawtypes")
        LinkedList list=new LinkedList(Arrays.asList(bookHistory.split("\,")));
//        if(list.contains(id))
//        {
//            list.remove(id);
//            list.addFirst(id);
//        }
//        else 
//        {
//            if(list.size()>=3)
//            {
//                list.removeLast();
//                list.addFirst(id);
//            }
//            else
//            {
//                list.addFirst(id);
//            }
//        }
         
        if(list.contains(id))
        {
            list.remove(id);
        }
        else if(list.size()>=3)
        {
            list.removeLast();
        }
        list.addFirst(id);
         
        StringBuffer sb=new StringBuffer();
        Iterator it=list.iterator();
        while(it.hasNext())
        {
            sb.append(id+",");
        }
        return sb.deleteCharAt(sb.length()-1).toString();
    }
 
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }
 
}