tcp服务器传送有关问题

tcp服务器传送问题
谁能帮我看看这个程序那里有错么！总是出现java.lang.NullPointerException错误

import java.io.*;
import java.net.*;
public class ServerClient 
{
public ServerClient(int port)
{
Server server=new Server(port);
server.start();
Client client=new Client(port);
client.start();
}
public static void main(String args[])
{
new ServerClient(7777);
}
}
class Server extends Thread
{
int port;
ServerSocket server;
Socket socket;
DataOutputStream outStream=null;
DataInputStream inStream=null;
public Server (int port)
{
try
{
this.port=port;
server=new ServerSocket(port);
}
catch(Exception e)
{
System.out.println(e.toString());
}
}
public void run()
{
try
{
socket=server.accept();
outStream=new DataOutputStream(socket.getOutputStream());
inStream=new DataInputStream(socket.getInputStream());
System.out.println("server is ok,please continue");
while(true)
{
String str=inStream.readUTF();
System.out.println("The server receive String:"+str);
outStream.writeUTF(str);
}
}
catch(Exception e)
{
System.out.println(e.toString());
}
}
}
class Client extends Thread
{
int port;
Socket socket;
DataInputStream inStream=null;
DataOutputStream outStream=null;
public Client(int port)
{
try
{
this.port=port;
socket=new Socket(InetAddress.getLocalHost(),port);
inStream=new DataInputStream(socket.getInputStream());
System.out.println("client is ok,please continue!");
}
catch(Exception e)
{
System.out.println(e.toString());
}
}
public void run()
{
try 
  { 
  while(true) 
  { 
  System.out.println("qingshuru");
  byte[] b=new byte[1024];
  String str="1"; 
  //从键盘读取字符串
  int m=System.in.read(b); 
  str=new String(b,0,0,m-1);
  //向服务器发送信息
  outStream.writeUTF(str); 
  //从服务器读取信息
  str=inStream.readUTF(); 
  System.out.println("The client receive String:"+str);
  } 
  } 
  catch(Exception e)
  { 
  System.out.println(e.toString());
  } 

}
}


------解决方案--------------------
在Client类的构造方法里面，要将outStream 变量初始化

Java code

public Client(int port)    {        try        {            this.port = port;            socket = new Socket(InetAddress.getLocalHost(), port);            inStream = new DataInputStream(socket.getInputStream());            outStream = new DataOutputStream(socket.getOutputStream());            //若outStream 不初始化，后面的程序对它的调用肯定会报空指针异常了            System.out.println("client is ok,please continue!");        } catch (Exception e)        {            e.printStackTrace();        }    }