请教!struts的购物车为什么不能添加商品??
这是shoppingCart的bean
public   class   ShoppingCart   {
private   double   totalPrice;
private   Map <Integer,Line>   lines=new   HashMap <Integer,Line> ();

public   void   addItem(Line   line){
Integer   bookId=line.getBook().getId();
if(lines.containsKey(bookId)){
Line   oldLine=lines.get(bookId);
int   oldQuantity=oldLine.getQuantity();
oldQuantity=+line.getQuantity();

oldLine.setQuantity(oldQuantity);
oldLine.setCost(oldLine.getLineCost());

lines.put(bookId,   oldLine);
}else{
line.setCost(line.getLineCost());

lines.put(bookId,   line);
}
}
}
jsp里面
<jsp:useBean   id= "cart "   class= "org.ysll.model.biz.entity.ShoppingCart "   scope= "session "> </jsp:useBean>
<%
List <Line>   lines=new   ArrayList <Line> ();
Map <Integer,Line> bookMap=cart.getLines();
Set <Integer> key=bookMap.keySet();
Iterator   it=key.iterator();


while(it.hasNext()){
Integer   i=(Integer)it.next();
Line   line=(Line)bookMap.get(i);
lines.add(line);
out.println(line.getBook().getName());
}
%>

<TABLE   cellSpacing=0   cellPadding=3   border=0>
<TBODY>
<!--   Header   Row   -->
<TR   bgColor=#cccccc>

<TD   noWrap   align=middle> <FONT   size=2> <B> 书号 </B> </FONT> </TD>
<TD   noWrap   align=middle> <FONT   size=2> <B> 书名 </B> </FONT> </TD>
<TD   noWrap   align=middle> <FONT   size=2> <B> 个数 </B> </FONT> </TD>
<TD   noWrap   align=middle> <FONT   size=2> <B> 价格 </B> </FONT> </TD>
<TD   noWrap   align=middle> <FONT   size=2> <B> 购买总价格 </B> </FONT> </TD>
</TR>

<%for   (int   i   =   0;   i   <   lines.size();   i++)   {
Line   l   =   lines.get(i);
%>
<TR   bgColor=#ffffff>


<TD   > <FONT   size=2> <B> <%=l.getBook().getId()%> <BR>
</B> </FONT> </TD>
<!--   name   and   description   for   item   -->
<TD   > <FONT   size=2> <B> <%=l.getBook().getName()%> </B> </FONT> </TD>

<!--   Qty   input   box   -->
<TD   > <FONT   size=2> <B> <%=l.getQuantity()%> </B> </FONT> </TD>


<!--   Unit   price   for   item   -->
<TD   > <FONT   size=2> <B> <%=l.getBook().getPrice()%> <BR>
</B> </FONT> </TD>

<!--   Extended   price   for   item   -->
<TD>
<TABLE   cellSpacing=2   cellPadding=3   border=0>
<TBODY>
<TR>
<TD   align=right   bgColor=#ffcc00> <FONT   size=2&g