Arraylist Improperly transfered through ObjectStreams

Question

code from client:
public void listen()
       String readtext;
       field.setText("");
       System.out.println("Starting to Listen");
       while(!socket.isClosed()){
                 try{
                dataType obj = (dataType)(in.readObject());
                switch(obj)
                     case String:
                     readtext = in.readUTF();
                     field.append(readtext);
                     System.out.println("Text recieved: " + readtext);
                     break;
                     case Userlist:
                     System.out.println("USERLIST UPDATE");
                     setUserList((Userlist)(in.readObject()));
                     break;
                        } catch (IOException e){}
                        catch(ClassNotFoundException cnfe){cnfe.printStackTrace();}
  private void setUserList(Userlist ul)
       System.out.println((ul.size()) + " Users");
       userlist.removeAll();
       for(int i = 0; i < ul.size(); i++)
       userlist.add(((User)(ul.get(i))).getName());
  }code from server's client threads:ClientWorker(Socket client, Server s) {
   this.client = client;
   this.server = s;
public void run(){
    String line;
    try{
      in = new ObjectInputStream(client.getInputStream());
      out = new ObjectOutputStream(client.getOutputStream());
    } catch (IOException e) {
      System.out.println("in or out failed");
      System.exit(-1);
    try{
     String userName = in.readUTF();
     This = new User(userName);
     server.UserNames.add(This);
     server.updateList();
     userset = true;
     } catch(IOException ieo){}
  ...code from server:  public void updateList()
       System.out.println((UserNames.size()) + " Users");
       for(int i = 0; i < Users.size(); i++)
       try{((ClientWorker)Users.get(i)).getOutputStream().writeObject(dataType.Userlist);
            ((ClientWorker)Users.get(i)).getOutputStream().writeObject(UserNames);
            catch(IOException e){e.printStackTrace();}
  }this code improperly prints the userlist (extends Arraylist) usernames object from the server to the clients
for one client, userlist usernames.size() = 1, works for both server and client...
for two clients:
client one:
userlist usernames.size() = 1 <- incorrect
client two:
userlist usernames.size() = 2 <- correct
it seems that after the first time the server sends the client the list that the list doesnt change on the client even though it is sending the same userlist object to all the clients

Answer 1

Your posted code is too fragmented to get clear understanding but if you are sending a same object repeatedly, try using writeUnshared() method instead of writeObject().