空指针异常问题

Question

把jdk编译时报的错误，和你的程序帖出来吧有很多原因可以造成空指针错误

Answer 1

大概程序出错在implements Runnable，我删除它后就没有问题了（但它是程序的必须模块）。如果加上后就说空指针错误，真的不明白啊！请指教啊：
public class ServerCenter implements Runnable
{
ServerSocket serverSocket;
Thread serverThread;

public ServerCenter()
{
try
{
serverSocket = new ServerSocket(5001);
serverThread=new Thread(this);
System.out.println("ServerCenter: server is started...");
serverThread.start();
}
catch(IOException e)
{
fail(e,"Could not start Server");
}
}
public static void fail(Exception e,String str)
{
System.err.println(str + " . " + e);
}
public void run()
{
try
{
while(true)
{
Socket postOffice = serverSocket.accept();

DataProcessingCenter con = new DataProcessingCenter(postOffice);
}
}
catch(IOException e)
{
  fail(e,"Not Listening");
}
}
public static void main(String args[])
{
new ServerCenter();
}
}
class DataProcessingCenter extends Thread
{
protected Socket postOffice;

protected ObjectInputStream acceptLetterMiss;
   
protected ObjectOutputStream sendLetterMan;

String response;
String findID;
String findID1;
String findID2;
String findID3;
String findFName;
String findLName;

liulan ll;
baozhuang bzh;
yunhuo yh;
shippingrate sr;
Recipient reci;
Orders orders;
public DataProcessingCenter(Socket postOffice)
{
try
{
acceptLetterMiss = new ObjectInputStream(postOffice.getInputStream());

sendLetterMan =new ObjectOutputStream(postOffice.getOutputStream());
}
catch(IOException e)
{
try
{
postOffice.close();
}
catch(IOException e1)
{
ServerCenter.fail(e1,"Unable to set up stream");
return;
}
}

this.start();
}
public void run()
{

try
{
while(true)
{
response = (String)acceptLetterMiss.readObject();
responseMiss();
}
}
catch(ClassNotFoundException e1)
{
ServerCenter.fail(e1,"" );
}
catch(IOException e2)
{
ServerCenter.fail(e2,"" );
}
finally
{
try
{
System.out.println("Socket closed");
postOffice.close();
}
catch(IOException e)
{
ServerCenter.fail(e,"");
}
}
}