空指针如何处理？java.lang.NullPointerException

Question

package com.muke;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;@SuppressWarnings("serial")public class LoginServlet extends HttpServlet{ public void doGet(HttpServletRequest request,HttpServletResponse response)throws IOException{ String piccode =(String)request.getSession().getAttribute("piccode"); String checkcode = request.getParameter("checkcode"); checkcode = checkcode.toUpperCase(); response.setContentType("text/html;chsrset=gbk"); PrintWriter out = response.getWriter(); if(checkcode.equals(piccode)){ out.println("输入正确"); } else{ out.println("输入有误"); } out.flush(); out.close(); }}

Answer 1

错误语句是 checkcode = checkcode.toUpperCase();