java为什么执行时说系统找不到指定的路径????如果("F:\\"))进入下一级目录就说拒绝访问？

package com.ming.url;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;

public class URLDemo {

public static void main(String[] args) throws IOException {
// TODO Auto-generated method stub
// 根据网址构造URL对象
URL url=new URL(
"http://image.baidu.com/detail/newindex?col=美女&tag=全部&pn=57&pid=11566540760&aid=401433457&user_id=790215689&setid=-1&sort=0&newsPn=&fr=&from=1");

System.out.println("主机名："+url.getHost());
System.out.println("资源路径："+url.getPath());
System.out.println("端口号："+url.getPort());
System.out.println("协议："+url.getProtocol());

// 通过URL打开连接
URLConnection conn=url.openConnection();
//截取文件名
String filename=url.getPath().substring(url.getPath().lastIndexOf("/"));
// 获取连接的输入流对象
BufferedInputStream bis=new BufferedInputStream(conn.getInputStream());
// 构造文件输出流对象
BufferedOutputStream bos=new BufferedOutputStream(new FileOutputStream("F:\\"));

byte[] bytes=new byte[1024*10];
int len=-1;
while((len=bis.read(bytes))!=-1){
bos.write(bytes,0,len);
bos.flush();
}
bos.close();
bis.close();
System.out.println("下载完成！");

}

}