(Collection<Book>)request.getAttribute("book")为什么不能转换？

改成这样
Collection c=(Collection)request.getAttribute("book");

楼主已经说过了,改为=(Collection)request.getAttribute("book"); 也不行

原代码是这样的：<%@ page language="java"  pageEncoding="gb2312"%>
<%@ taglib prefix="c" uri="/WEB-INF/c.tld"%>
<%@ page import="java.util.*" %>
<%@ page import= "shiyin.bookShop.bean.*"%>
<%@ page import= "shiyin.bookShop.shopcart.*"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
  <link href="css/mycss.css" rel="stylesheet" type="text/css"/>

    <title>浏览图书</title>
  </head>
  <body leftmargin=0 topmargin="0"  bottommargin="0" marginwidth="0" marginheight="0">
  <div class="top"></div>
  <div class="main1"></div>
  <div class="main2"><center>
  <p> </p>
  <p>
  　<a href="<%=request.getContextPath() %>/querybook.jsp">继续查询</a>
　　<a href="<%=request.getContextPath() %>/showcart.jsp">查看购物车</a>
  </p>
  <table border="1">
<%
   if(!request.getAttribute("type").equals("BY_ISBN"))
   {
   Collection<Book> books =(Collection<Book>)request.getAttribute("book");
   Iterator<Book> ibooks=books.iterator();
   while(ibooks.hasNext()){
   Book book=ibooks.next();

%>
<tr>
<td><%=book.getIsbn() %></td>
<td><%=book.getTitle() %></td>
<td><%=book.getAuthor() %></td>
<td><%=book.getPrice() %></td>
<td><a href="shoppingCart?operation=addItem&isbn=<%=book.getIsbn() %>">加入购物车</a></td>
</tr>
<%
}
%>
  </table>
<%
}else{
BookDetails book=new BookDetails();
book=(BookDetails)request.getAttribute("book");
if(book==null){%>
<span class="bodytxt">对不起，本书店没有你要求的书</span><%=request.getAttribute("key") %>
<%
return;
  }
  else{
%>
  <p class="bodyh1">你的查询结果是：             </p>
  <table border=1>
         <tr>
           <td class="bodytxt">ISBN</td>
           <td class="bodytxt">书名</td>
           <td class="bodytxt">作者</td>
           <td class="bodytxt">价格</td>
         </tr>
         <tr>
         <c:set var="bookItem" value="${requestScope.book}"/>
          <td><c:out value="${bookItem.isbn}" /></td>
          <td><c:out value="${bookItem.title}" /></td>
          <td><c:out value="${bookItem.author}" /></td>
          <td><c:out value="${bookItem.price}" /></td>
          <td><a href="shoppingcart?operation=addItem&isdn=<c:out value="${bookitem.isbn}" />">加入购物车</a></td>
         </tr>
   </table>
   <%
   }
   }
    %>
  </center>
  <div class="txt">2005-2008</div>
  </div>

  <div class="main3"></div>

  </body>
</html>

报错是这个：org.apache.jasper.JasperException: Exception in JSP: /browse.jsp:2421:   </p>
22:   <table border="1">
23: <%
24:    if(!request.getAttribute("type").equals("BY_ISBN"))
25:    {
26:    Collection<Book> books =(Collection<Book>)request.getAttribute("book");
27:    Iterator<Book> ibooks=books.iterator();
java.lang.NullPointerException
org.apache.jsp.browse_jsp._jspService(browse_jsp.java:90)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:331)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:329)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
shiyin.bookShop.servlet.loginServlet.doBrowse(loginServlet.java:67)
shiyin.bookShop.servlet.loginServlet.doLogin(loginServlet.java:54)
shiyin.bookShop.servlet.loginServlet.doPost(loginServlet.java:31)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
shiyin.bookShop.util.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:18)

LZ,
if(!request.getAttribute("type").equals("BY_ISBN"))
type是什么?你改成request.getParameter("type");这样看看

if(!request.getAttribute("type").equals("BY_ISBN"))
改成if(!"BY_ISBN".equals(request.getAttribute("type"))如果不行，改成if(!"BY_ISBN".equals(request.getParameter("type"))

我的那个“tpye”servlet那里传过来的　servlet中有这样的代码
　　　request.setAttribute("book", booklist);
request.setAttribute("key", varkey);
request.setAttribute("type",vartype);

我把代码if(!request.getAttribute("type").equals("BY_ISBN"))改成
if(!"BY_ISBN".equals(request.getParameter("type"))
可以运行了，但是那个Collection<Book> books =(Collection<Book>)request.getAttribute("book");
警告还是还有，jsp的request.getAttribute("book");就是不能强制转换为泛型。还什么办法可以弄呢？

那个警告你就别指望能去掉了。这个问题讨论过很多次。OVER.http://topic.csdn.net/u/20080123/03/1e041618-c3f7-493b-b257-d0447860ac0f.html

你可以这样试试看if(!request.getParameter("type").equals("BY_ISBN")){
          Collection books =(Collection)request.getAttribute("book");
          Iterator ibooks=books.iterator();
          while(ibooks.hasNext()){
              Book book=(Book)ibooks.next();
          }
}

调试易

(Collection<Book>)request.getAttribute("book")为什么不能转换？

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