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Question

if(submit==null)
addItem(item);
if(submit.equals("add")) >>>>少else
addItem(item);
else if(submit.equals("remove"))
removeItem(item);

Answer 1

cart.getItems() 可能为空，你把它可能为空的情况处理一下

Answer 2

你的addItem(item)参数item总是null

Answer 3

cart.processRequest(request);有问题，将processRequest变成静态方法

Answer 4

public void processRequest(HttpServletRequest request) {
=====================================
if(submit==null)
addItem(item);
               
如果submit是null的话这里应该结束了，不该继续，所以nullexception
应该修改为：         
                  { 
                      addItem(item);
                      return;
                  }
=================================
if(submit.equals("add"))
addItem(item);
else if(submit.equals("remove"))
removeItem(item);
reset();
}

Answer 5

public String [] getItems() 
{
     if (v.size()>0){
String [] s=new String[v.size()];
v.copyInto(s);
return s;
     }
}

Answer 6

public void processRequest(HttpServletRequest request) {
                if(submit==null)
                {
                    addItem(item);
                }
                else if(submit.equals("add"))
                {
                    addItem(item);
                }
                else if(submit.equals("remove"))
                {
                    removeItem(item);
                }
                else reset();
        }其余的地方你自己看看。