Question

So I have a server-client combo which is just supposed to pass a custom Object back and forth. I’ using the ObjectInputStream and ObjectOutpustStream classes to achieve this.

Here is the server’s loop:

while((inputPosition = (Vector2i) objectIn.readObject()) != null) {
    Print.log("input line: " + inputPosition.toString());
    outputLine = "you moved to " + inputPosition.toString();
    out.println(outputLine);        
}

Where inputPosition is a Vector2i, a simple class that just holds 2 integers, x and y.

Here is the client’s loop:

while((serverOutput = in.readLine()) != null) {
    Print.log("Server says: " + serverOutput);
    position = calculatePosition(position, reader);
    Print.log("sending over: " + position.toString());
    objectOut.writeObject(position);
}

The calculate position method just looks like this:

private static Vector2i calculatePosition(Vector2i position, BufferedReader reader) throws IOException {
    Print.log("i just got this: " + position.toString());
    String entry = reader.readLine().substring(0, 1);
    if(entry.equals("w"))
        position.y++;
    else if(entry.equals("s"))
        position.y--;
    else if(entry.equals("a"))
        position.x--;
    else if(entry.equals("d"))
        position.x++;

    return position;
}

So here is what happens. I connect to the server, and after moving one coordinate successfully, it just get’s stuck on the same coordinate over and over:

Server says: Use wasd to move around.
i just got this: 5, 5
w
sending over: 5, 6
Server says: you moved to 5, 6
i just got this: 5, 6
w
sending over: 5, 7
Server says: you moved to 5, 6
i just got this: 5, 7
a
sending over: 4, 7
Server says: you moved to 5, 6
i just got this: 4, 7

You can see in the “sending over” line that the vector2i object on the client side is up-to-date, but the response I get from the server is just the same one over and over. There server’s log looks like this:

input line: 5, 6
input line: 5, 6
input line: 5, 6

It seems to be receiving the same data over and over, but according to my log, the client should be sending new data over.

Does anyone have an idea what I did wrong?

Answer 1

After you have sent an object once, it send a reference to that object. This means

• if you mutate an object and send it again, you won’t see the changes
• the Object Stream’s memory will grow and grow as it needs to retain a reference to every object which has been sent/received.

The way to avoid both these issue is to call reset() regularly. The library can’t do this for you as it doesn’t know what it can be safely done.