麻烦老师帮我看一下代码，是上节课编程题，不知道错在哪，谢谢

package com.imooc.servlet;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class ComputerServlet extends HttpServlet {

public int add(int number) {

int sum = 0;

for(int i = 1;i<=number;i++)

{

sum += i;

}

return sum;

}

public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException,IOException

{

String num = request.getParameter("num");

int n = Integer.parseInt(num);

PrintWriter out = response.getWriter();

out.println(add(n));

}

}

<servlet>

<servlet-name>computer</servlet-name>

<servlet-class>com.imooc.servlet.ComputerServlet</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>computer</servlet-name>

<url-pattern>/ComputerServlet</url-pattern>

</servlet-mapping>