NullPointerException该怎么解决

sunlitpalm 2007-02-14 02:26:25

<%@ page language="java" %>
<%@ page session="true" %>
<%@ page contentType="text/html;charser=gb2312"%>
<%@ page import="java.lang.*"%>
<%@ page import="java.util.*"%>
<%!
public String getMessage(int[] result){
switch(result[0]){
case -2:return "san wei shu";
case -1:return "ge shi wrong!";
case 0:return "right";
default :return "go on!";
}
}
public int[] checkNumber(String guessnum,String answernum){
if(guessnum.length()!=3)
return new int[]{-2,0,0};
int num=0;
try{
num=Integer.parseInt(guessnum);
}catch(NumberFormatException e){
return new int[]{-1,0,0};
}
int a=0;
int b=0;
for(int j=0;j<3;j++){
int pos=answernum.indexOf(guessnum.substring(j,j+1));
if(pos==j)
a++;
else if(pos>=0)
b++;
}
if(a==3)
return new int[]{0,a,b};
else
return new int[]{1,a,b};
}
%>
<%
Vector history=(Vector)session.getValue("HISTORY");
String answernum=(String)session.getValue("ANSWER");
String guessnum=null;
String message=null;
boolean gameover=false;
if(history==null||answernum==null){
history=new Vector();
answernum="456";
session.putValue("HISTORY",history);
session.putValue("ANSWER",answernum);
}else{
guessnum=request.getParameter("guess_number");
int[] result=checkNumber(guessnum,answernum);
message=getMessage(result)+"ni cai de shi"+guessnum;
if(result[0]==0)
gameover=true;
else if(history.size()>=4){
gameover=true;
message+="answer:"+answernum;
}
history.insertElementAt(message,0);
session.putValue("ANSWER",answernum);
}
if(gameover){
session.removeValue("HISTORY");
session.removeValue("ANSWER");
}
%>
<html>
<title>
a example of session
</title>
<body bgcolor="#ffffff">
<h1>cai shu zi!</h1>
<form name="form1" method="get" action="numguess1.jsp">
number:<input type="text" name="guess_number" size="4" maxlength="4">
<input type="submit" name="submit" value="enter">
</form>
<%
int i=history.size();
for(Enumeration e=history.elements();e.hasMoreElements();){
%>
<%=i%>,<%=(String)e.nextElement()%><br>
<%
i--;
}
if(gameover){
%>
<form name="form2" method="get" action="numguess1.jsp">
<input type="submit" name="submit" value="go on">
</form>
<%
}
%>
</body>
</html>

错误提示：
type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

org.apache.jasper.JasperException
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:358)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:301)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:248)
javax.servlet.http.HttpServlet.service(HttpServlet.java:856)

root cause

java.lang.NullPointerException
org.apache.jsp.exam.numguess1_jsp.checkNumber(numguess1_jsp.java:22)
org.apache.jsp.exam.numguess1_jsp._jspService(numguess1_jsp.java:91)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:133)
javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:311)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:301)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:248)
javax.servlet.http.HttpServlet.service(HttpServlet.java:856)

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