编译都没错就是运行出错，第12行有问题吗？

Question

org.apache.jasper.JasperException: Exception in JSP: /bookstore.jsp:129: <%@ include file="head.html"%>
10: 
11: <table>
12: <%for(ListIterator iter=bookDB.getBooks().listIterator();iter.hasNext();){
13:  BookDetails book=(BookDetails)iter.next();
14: %>
15:     <tr>    
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:491)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:413)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:308)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:259)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
root cause java.lang.NullPointerException
database.BookDB.getBooks(BookDB.java:35)
org.apache.jsp.bookstore_jsp._jspService(bookstore_jsp.java:87)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:371)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:308)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:259)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)

Answer 1

补充：
Eclipse中代码中黄线提示：
“ListIterator 是原始类型。应该将对通用类型 ListIterator<E> 的引用参数化”
求问经验十足的高人咋回事？

Answer 2

java.lang.NullPointerException
database.BookDB.getBooks(BookDB.java:35)检查你自己的Java代码BookDB.java第35行，这里空指针了。
黄线可以暂不管，是因为你没有用泛型。

Answer 3

bookDB.getBooks()空指针异常，
可能bookDB == null
也可能bookDB.getBooks() == null
你自己排查一下

Answer 4

bookDB是空指针  ，java.lang.NullPointerException
database.BookDB.getBooks(BookDB.java:35)

Answer 5

description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException: database.BookDetails cannot be cast to java.lang.Comparable
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:500)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:413)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:308)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:259)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
root cause java.lang.ClassCastException: database.BookDetails cannot be cast to java.lang.Comparable
java.util.Arrays.mergeSort(Arrays.java:1144)
java.util.Arrays.sort(Arrays.java:1079)
java.util.Collections.sort(Collections.java:117)
database.BookDBAO.getBooks(BookDBAO.java:79)
database.BookDB.getBooks(BookDB.java:35)
org.apache.jsp.bookstore_jsp._jspService(bookstore_jsp.java:87)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:371)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:308)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:259)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
这又是啥情况？

Answer 6

debug 看一下 或者system.out看下 哪个值是null就好了哇
呵呵

Answer 7

java.lang.ClassCastException: database.BookDetails cannot be cast to java.lang.ComparableBookDetails 这个类要implements Comparable这个接口的哇

Answer 8

谢谢，果然实现了implements Comparable就行了。