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服務器-出現下面錯誤怎麼解決啊，求方法，請大神們幫我看看！

編輯：編程綜合問答

出現下面錯誤怎麼解決啊，求方法，請大神們幫我看看！

if(agent.contains("MSIE")){

這句報錯，錯誤提示The method contains(CharSequence) from the type String refers to the missing type CharSequence

 package cn.itcast.response;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URLEncoder;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 文件下載
 * @author Administrator
 *
 */
public class DownloadServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 先獲取到文件，讀入輸入流中
        // 獲取文件的絕對磁盤路徑
        String path = getServletContext().getRealPath("/img/外國美女.jpg");
        // 包含文件的名稱    c:\tomcat\webapps\day10\img\girl7.jpg
        // 截取文件的名稱    girl7.jpg
        String filename = null;
        // 獲取最後一個\的位置
        int index = path.lastIndexOf("\\");
        if(index != -1){
            filename = path.substring(index+1);
        }

        // 判斷是什麼浏覽器 Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)
        // Mozilla/5.0 (Windows NT 6.1; WOW64; rv:30.0) Gecko/20100101 Firefox/30.0
        String agent = request.getHeader("User-Agent");
        // System.out.println(agent);
        if(agent.contains("MSIE")){
            filename = URLEncoder.encode(filename,"UTF-8");
        }

        if(filename != null){
            // 設置頭信息
            response.setHeader("Content-Disposition", "attachment;filename="+filename);

            System.out.println(request.getRemoteAddr());

            System.out.println(path);
            InputStream in = new FileInputStream(path);
            // 通過response讀出到客戶端
            OutputStream os = response.getOutputStream();
            // io的拷貝
            byte [] b = new byte[1024];
            int len = 0;
            while((len =  in.read(b)) != -1){
                os.write(b, 0, len);
            }
            in.close();
            // os.close();服務器管理流
        }


    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

}