Server/client IO ArrayList

Question

So, I have this client and server classes which work like that� Server just starts and waits for a client to connect, then the client �sends� a String which represents a username for that client. Server then stores the username in an ArrayList called usersList. It also stores the ObjectOutputStream(the server side ObjectOutputStream) of this connection in a HashMap called outStreamMap and then sends back to every connected client that ArrayList. The code is:
Server:
import java.net.*;
import java.io.*;
import java.util.*;
public class Server{
public HashMap<String,ObjectOutputStream> outStreamMap;
public ArrayList<String> usersList;
public Server(){
  usersList=new ArrayList<String>();
  outStreamMap=new HashMap<String,ObjectOutputStream>();
  CallReceiver cr=new CallReceiver();
public static void main(String[] args){
  Server serv=new Server();
class CallReceiver implements Runnable{
ServerSocket serverSocket;
public CallReceiver(){
  try{
   serverSocket=new ServerSocket(5432);
   Thread callReceiverThread=new Thread(this);
   callReceiverThread.start();
  }catch(IOException e){e.printStackTrace();}
public void run(){
  while(true){
   try{
    Socket socket=serverSocket.accept();
    InputStream is=socket.getInputStream();
    OutputStream os=socket.getOutputStream();
    ObjectOutputStream objOutStr=new ObjectOutputStream(os);
    ObjectInputStream objInStr=new ObjectInputStream(is);
    Object obj=objInStr.readObject();//reads the username
    String username=(String)obj;
    usersList.add(username);//adds the user to the ArrayList
    outStreamMap.put(username,objOutStr);//adds the output stream to the Map
    for(String user:usersList){//sends the ArrayList to every user
       outStreamMap.get(user).writeObject(usersList);
       outStreamMap.get(user).flush();
       System.out.println("Sent ArrayList:"+usersList+" to:"+user);
   }catch(Exception e){
      e.printStackTrace();}
}//end of CallReceiver
}//end of server
Client:
import java.net.*;
import java.io.*;
import java.util.*;
public class Client{
public Client(){
  try{
   Socket socket=new Socket("127.0.0.1",5432);
   OutputStream outStr=socket.getOutputStream();
   InputStream inStr=socket.getInputStream();
   ObjectOutputStream objOutStr=new ObjectOutputStream(outStr);
   ObjectInputStream objInStr=new ObjectInputStream(inStr);
   objOutStr.writeObject("John");//sends a string representing a username
   objOutStr.flush();
   ArrayList userlist;
   while(true){
     Object obj=objInStr.readObject();//reads an object wich actually is an ArrayList
     userlist=(ArrayList)obj;
     System.out.println("ArrayList received: "+userlist);
  }catch(Exception e){e.printStackTrace();}
public static void main(String[] args){
Client cl=new Client();
}The client all it does is just waiting for an ArrayList and then just prints it on the command line.
Lets say for example that you run a client with username, john. You get on the command line something like:
ArrayList received: [john]
Then you connect a second client with username, george (the first client is still connected, offcourse).
So, you get on the first client�s command line:
ArrayList received: [john]
And on the second client�s command line:
ArrayList received:[john, george]
And so it goes, if you connect another one you get on the first�s [john], on the second�s [john, george] and on the third�s [john, george, mike]. And this is the problem I cannot understand how this is happening since the server �sends� the exact same ArrayList object to all clients! Please someone more openeyed than me help me!!

Answer 1

This is due to ObjectStream caching, instead of resending the data, it will only send a marker (as in 'object foo sent again'). It doesn't know that the contents of the object are changed.
This is due to the fact that ObjectStreams aren't really supposed to be used this way, and this way if you're writing the same object many times, it'll save on the bandwidth (it'll send only the backreference).
Look at the writeUnshared() method in ObjectOutputStream.