空指针错误有关问题
日期：2014-05-20 浏览次数：20718 次
空指针异常问题
这是jsp页面的代码
<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP 'jspShopCar.jsp' starting page</title>
</head>
<body>
<h3>选择所需的商品</h3>
<form name = "form1" action = "jspShopCar.jsp" method = "post">
<select name = "item">
<option>ASP.NET</option>
<option>C++</option>
<option>C语言</option>
<option>THINGING IN JAVA </option>
<option>ORCALE</option>
<option>SQL/PLUS</option>
</select>
<input type = "submit" name = "submit" value = "Add">
<input type = "submit" name = "submit" value = "Remove">
</form>
</body>
</html>
<jsp:useBean id = "beanCar" scope = "session" class = "chap7.ex.ShoppingCar"/>
<jsp:setProperty name = "beanCar" property = "*"/>
<%
String tmpbtn = request.getParameter("submit");
String tmpname = request.getParameter("item");
if(tmpbtn!=null){
out.print("<hr size = '1' noshade = 'noshade' />");
out.print("<h4>购物车中的商品:</h4>");
}
if(tmpbtn.equals("Add")){
beanCar.AddItem(tmpname);
}
if(tmpbtn.equals("Remove")){
beanCar.RemoveItem(tmpname);
}
out.print("<ol>");
String[] item = beanCar.getItems();
for(int i = 0;i<item.length;i++){
out.print("<li>" +item[i] +"</li>");
}
out.print("</ol>");
beanCar.reset();
%>
==============================================
这个是bean的代码
package chap7.ex;
import java.util.*;
public class ShoppingCar {
Vector v = new Vector();
String submit = null;
String item = null;
public void AddItem(String name){
v.addElement(name);
}
public void RemoveItem(String name){
v.removeElement(name);
}
public void setItem(String name){
item = name;
}
public void setSubmit(String s){
submit= s;
}
public String[] getItems(){
String[] s = new String[v.size()];
return s;
}
public void reset(){
item = null;
submit = null;
}
}
我一执行jsp就会出现空指针异常，调了好几天了，大家帮帮忙吧
------解决方案--------------------
if(tmpbtn.equals("Add")){
beanCar.AddItem(tmpname);
}
if(tmpbtn.equals("Remove")){
beanCar.RemoveItem(tmpname);
}
改if("Add".equals(tmpbtn)){
beanCar.AddItem(tmpname);
}
if("Remove".equals(tmpbtn)){
beanCar.RemoveItem(tmpname);
}
如果tmpbtn获取是null值，那是用equals方法的时候当然会出现npe
这是jsp页面的代码
<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP 'jspShopCar.jsp' starting page</title>
</head>
<body>
<h3>选择所需的商品</h3>
<form name = "form1" action = "jspShopCar.jsp" method = "post">
<select name = "item">
<option>ASP.NET</option>
<option>C++</option>
<option>C语言</option>
<option>THINGING IN JAVA </option>
<option>ORCALE</option>
<option>SQL/PLUS</option>
</select>
<input type = "submit" name = "submit" value = "Add">
<input type = "submit" name = "submit" value = "Remove">
</form>
</body>
</html>
<jsp:useBean id = "beanCar" scope = "session" class = "chap7.ex.ShoppingCar"/>
<jsp:setProperty name = "beanCar" property = "*"/>
<%
String tmpbtn = request.getParameter("submit");
String tmpname = request.getParameter("item");
if(tmpbtn!=null){
out.print("<hr size = '1' noshade = 'noshade' />");
out.print("<h4>购物车中的商品:</h4>");
}
if(tmpbtn.equals("Add")){
beanCar.AddItem(tmpname);
}
if(tmpbtn.equals("Remove")){
beanCar.RemoveItem(tmpname);
}
out.print("<ol>");
String[] item = beanCar.getItems();
for(int i = 0;i<item.length;i++){
out.print("<li>" +item[i] +"</li>");
}
out.print("</ol>");
beanCar.reset();
%>
==============================================
这个是bean的代码
package chap7.ex;
import java.util.*;
public class ShoppingCar {
Vector v = new Vector();
String submit = null;
String item = null;
public void AddItem(String name){
v.addElement(name);
}
public void RemoveItem(String name){
v.removeElement(name);
}
public void setItem(String name){
item = name;
}
public void setSubmit(String s){
submit= s;
}
public String[] getItems(){
String[] s = new String[v.size()];
return s;
}
public void reset(){
item = null;
submit = null;
}
}
我一执行jsp就会出现空指针异常，调了好几天了，大家帮帮忙吧
------解决方案--------------------
if(tmpbtn.equals("Add")){
beanCar.AddItem(tmpname);
}
if(tmpbtn.equals("Remove")){
beanCar.RemoveItem(tmpname);
}
改if("Add".equals(tmpbtn)){
beanCar.AddItem(tmpname);
}
if("Remove".equals(tmpbtn)){
beanCar.RemoveItem(tmpname);
}
如果tmpbtn获取是null值，那是用equals方法的时候当然会出现npe
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