首先创建一个calculate.jsp

这是用Javascript代码来验证，代码如下：

<script type="text/javascript">

	function check(){
	if(form.n1.value==""&&form.value.n2==""){
	window.alert("值不能为空");
	return false;
	}
	if(isNaN(form.n1.value)&&isNaN(form.n2.vavlue)){
	window.alert("请确认输入的是否是数字！");
	return false;
	}
	if(form.operator.value=="/"&&form.n2.value=="0"){
	window.alert("除数不能为0,请重新输入！ ");
	return false;
	}
	}
	</script>

使用Java代码来计算,代码如下：

<%
 String operator=request.getParameter("operator");
 int n1=Integer.parseInt(request.getParameter("n1"));
 int n2=Integer.parseInt(request.getParameter("n2"));
 int result=0;
   if("+".equals(operator)){
         	 result= n1+n2;
           }
           else if("-".equals(operator)){
          	 result=n1-n2;
           }
           else if("*".equals(operator)){
          	 result=n1*n2;
           }
           else {
          	 result=n1/n2;
           }
 			 request.setAttribute("n1", n1); 
		     request.setAttribute("n2", n2); 
 			request.setAttribute("result", result);  //往request作用域里放一个值
           	request.getRequestDispatcher("calculate.jsp").forward(request,response); 
       %>

form表单来写一个简单的页面，代码如下：

<form name="form" action="calculate_do.jsp" method="post" >
   <input name="n1" value="<%= request.getAttribute("n1")==null?"":request.getAttribute("n1") %>">
   <select name="operator">
   <option value="+" <%= "+".equals(operator)?"selected":"" %>>+</option>
   <option value="-" <%= "-".equals(operator)?"selected":"" %>>-</option>
   <option value="*" <%= "*".equals(operator)?"selected":"" %>>*</option>
   <option value="/" <%= "/".equals(operator)?"selected":"" %>>/</option>

   </select>
   <input name="n2" value="<%= request.getAttribute("n2")==null?"":request.getAttribute("n2") %>">
   <input type="submit" value="=" onclick="return check();" >
  <input name="result"   value="<%= request.getAttribute("result")==null?"":request.getAttribute("result") %>">  
  </form>

源代码：

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">

    <title>My JSP 'calaulate.jsp' starting page</title>

	<meta http-equiv="pragma" content="no-cache">
	<meta http-equiv="cache-control" content="no-cache">
	<meta http-equiv="expires" content="0">    
	<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
	<meta http-equiv="description" content="This is my page">
	<script type="text/javascript">

	function check(){
	if(form.n1.value==""&&form.value.n2==""){
	window.alert("值不能为空");
	return false;
	}
	if(isNaN(form.n1.value)&&isNaN(form.n2.vavlue)){
	window.alert("请确认输入的是否是数字！");
	return false;
	}
	if(form.operator.value=="/"&&form.n2.value=="0"){
	window.alert("除数不能为0,请重新输入！ ");
	return false;
	}
	}
	</script>  </head>

  <body>
  <%
 String operator=request.getParameter("operator");
 int n1=Integer.parseInt(request.getParameter("n1"));
 int n2=Integer.parseInt(request.getParameter("n2"));
 int result=0;
   if("+".equals(operator)){
         	 result= n1+n2;
           }
           else if("-".equals(operator)){
          	 result=n1-n2;
           }
           else if("*".equals(operator)){
          	 result=n1*n2;
           }
           else {
          	 result=n1/n2;
           }
 			 request.setAttribute("n1", n1); 
		     request.setAttribute("n2", n2); 
 			request.setAttribute("result", result);  //往request作用域里放一个值
           	request.getRequestDispatcher("calculate.jsp").forward(request,response); 
       %>


   <form  name="form" action="calculate_do.jsp" method="post"  >
   <input name="n1" value="<%= request.getAttribute("n1")==null?"":request.getAttribute("n1") %>">
   <select name="operator">
   <option value="+" <%= "+".equals(operator)?"selected":"" %>>+</option>
   <option value="-" <%= "-".equals(operator)?"selected":"" %>>-</option>
   <option value="*" <%= "*".equals(operator)?"selected":"" %>>*</option>
   <option value="/" <%= "/".equals(operator)?"selected":"" %>>/</option>

   </select>
   <input name="n2" value="<%= request.getAttribute("n2")==null?"":request.getAttribute("n2") %>">
   <input type="submit" value="=" onclick="return check();" >
  <input name="result"   value="<%= request.getAttribute("result")==null?"":request.getAttribute("result") %>">  
  </form>


  </body>
</html>