为什么收不到文件？？
服务器端

public   class   FileServer
{
        public   FileServer()
        {
                try
{
ServerSocket   ss   =   new   ServerSocket(4700);
System.out.println   ( "服务器启动！ ");
while   (true)  
{
Socket   socket   =   ss.accept();
BufferedReader   in=new   BufferedReader(new   InputStreamReader(socket.getInputStream()));
PrintWriter   out=new   PrintWriter(socket.getOutputStream());

FileInputStream   fis=new   FileInputStream( "E:\\1.txt ");
FileOutputStream   fos=null;
byte[]   buffer=new   byte[1024];
int   n=0;

while((n=fis.read(buffer))!=-1)
{
fos.write(buffer,0,n);
}
out.println(buffer);  
}
}
catch   (Exception   e)
{
e.printStackTrace();
}
}

              public   static   void   main(String[]   args)
              {
new   FileServer();
}
}

客户端：
public   class   FileClient
{
public   FileClient()
{
Socket   socket   =   null;
BufferedReader   in   =   null;
PrintWriter   out   =   null;
FileInputStream   fis   =   null;
FileOutputStream   fos   =   null;
byte[]   buffer   =   new   byte[1024];
int   n   =   0;

try
{
socket   =   new   Socket( "127.0.0.1 ",4700);
System.out.println( "连接成功！ ");
in   =   new   BufferedReader(new   InputStreamReader(socket.getInputStream()));
out=new   PrintWriter(socket.getOutputStream());

fos   =   new   FileOutputStream( "F:\\1.txt ");
while((n=fis.read(buffer))   !=   -1)*******NullPointerException************
{
fos.write(buffer,0,n);
}
}
catch(Exception   e)
{
e.printStackTrace();
}
}

public   static   void   main(String[]   args)
{
new   FileClient();
}
}
我哪写错了？为什么会出现NullPointerException?希望高手帮忙！

------解决方案--------------------
NullPointerException详细一点的信息？哪个Java文件的哪行？那一行是什么代码？
------解决方案--------------------
fis为null
------解决方案--------------------
FileInputStream fis = null; <-------这个fis一直是null，如下修改即可
------------------------
InputStream in = socket.getInputStream();
out=new PrintWriter(socket.getOutputStream());

fos = new FileOutputStream( "F:\\1.txt ");
while((n=in.read(buffer)) != -1)
...