Difference in information sent / received using externalisable classes

Question

I am a little unsure whether this is the right place to post, I am relatively new to Java but trying hard to learn it well. However this is quite a specific problem with a personal (for fun) project I have been undertaking and it is driving me slowly insane.
This is a bit complicated for me to simple post swathes of code so I will attempt to explain. Any help is appreciated as this has been infuriating me for 2-days now.
Essentially I have made a small game for multiple clients connecting to a server. The client can send a request to move from a position to another to the server and the server responds by moving them and sending them a list of other players at that location, however for some reason at certain points the server will write out what I believe is the correct information to the client but the client will read in something completely different.
Here is the code which sends the "node" (containing the list of players as well as other information).
Packet packet = Packet.factoryCreateMovementArrived(publicNode);
server.sendPacketToClient(playerID, otherPacket);
public void sendPacketToClient(int playerID, Packet packet) throws IOException
     synchronized(playerIDToOutput)
          ObjectOutputStream obOut = (ObjectOutputStream)playerIDToOutput.get(playerID);
          obOut.writeObject(packet);
}I wrote my own "packet" class for this which implements the externalizable interface, I factory a certain type of packet and when writing out use a switch to decide what information will be sent. Below is the code for that type of packet (note: the node is also externalisable with a similar method, also included).
case TYPE_MOVEMENT_ARRIVED:
     out.writeObject(locationNode);
     break;
case TYPE_PUBLIC_NODE_UNDOCKED:               
     out.writeInt(type);
     out.writeObject(nodeID);
     out.writeObject(shipsSpace);I have examined the packets I am sending out from the server and they contain what I expect however what the client reads in, seems completely different at times and I cannot for the life of me figure out why.
Here for example are the methods to read in the packet and the node:
case TYPE_MOVEMENT_ARRIVED:
     this.locationNode = (Node)in.readObject();
        break;
case TYPE_PUBLIC_NODE_UNDOCKED:                    
     this.type = in.readInt();
     this.nodeID = (String)in.readObject();
     this.shipsSpace = (Vector)in.readObject();And this is used by this method to read in the packet when received from the server:
while(true)
     try
          Packet messagePacket = (Packet)obIn.readObject();                         
          dealWithPacket(messagePacket);                         
private void dealWithPacket(Packet packet)
     if (packet.getType() == Packet.TYPE_MOVEMENT_ARRIVED)
          Node toNode = packet.getLocationNode();
          active.arrivedAtLocation(toNode);
     }However by the time it gets to there the data is wrong, the list of people at that node seems to be similar to when the client first logs on. I have debugged my way through the code numerous times and although the server seems to be creating packets containing the correct and current list of players when received at the other end it is simply the list for that node at the beginning of the game. I do not know whether it is possible that although the packet contains the correct list somehow I am sending a different one, but I cannot see how. The same for when constructing the packet with the read methods. .. whether somehow I am not actually reading in what I have sent..
It's hard for me to explain properly and I cannot exactly post all my code because there's a fair amount of it, however I have been over and over what I have written checking that I am not simply referencing something else, I even wrote the packets I was sending from the server into a file and read that in in another program I wrote and examined them, they were correct! It is infuriating.
Any help would be greatly appreciated, even simply ideas of how to test this further would be welcome.
Look forward to your responses and apologise for any obvious errors :)

Answer 1

So when I'm writing to a stream objects I have already written can essentially be referenced / reused and sent again? I didn't know that but if that's what is happening it explains alot.
So as I'm sending the same node when you move back to that position it's already in the stream so it simply uses that version? Sorry for asking further questions I just want to make sure I understand what's happening (I went and read the API.. just checking).
And no warnerja it wasn't a "please simply wave an magic wand and write the project for me", I've seen posts like that from other people. I have written alot of code around this and I simply did not want to post it all. If reseting my output stream solves the problem then someone clearly understood my dilema even if you did not. A more helpful post might have been "You haven't specified enough for me to understand the problem please specify the following ... a) .. b)" etc. Don't simply moan at me.
Thanks DrClap.