Getting objects to move between PCs

Question

After CS 150, I find myself back in the world of Java. I recently learned a neat language called Processing (a.k.a. Proce55ing), and I am one of the coders (and am way rusty to be of much use at this point) for a really cool project.
I'm wondering if I can nag you with some questions on java.net.*. I need to get some objects passed around between 3 PCs. I need to know about latency issues. Basically, we've got Java doing OpenGL graphics on 6 projectors powered by 3 computers. Imagine each PC having a really wide screen... when a autonomous creature gets to the edge of one PC, it needs to continue its thang on the next PC... move fluidly.
I've checked out some Java.net.* sample code on sun's site. Specifically their KnockKnock Server. However, its multithread support is for interacting with clients on a one-on-one basis... not like a chat-room style environment. So, I don't even have any sample code for a ultra-basic Java Chat Room.... nor do I know how to pass anything but a string to the server, or to the client.
There are ways to use my limited knowledge to do what I want to do... just pass a string, and have the server store that into a variable which is accessible by every server thread. Then, create a loop in main that keeps searching through that array of strings to pass them off to the PC... the string would have the coordinate information of the autonomous creature, and create a new one of that object with those Vec3f point. However, latency is a key issue, and it would be easiest if I could just pass an object to the server (in this case: a SimpleVehicle) along with an intended target computer, and then that computer would get that vehicle to add to its array of SimpleVehicles. The important thing is speed, so it might be best to have every computer have a client<->server connection with each other... I get utterly baffled reading the Java Docs. But, I seem to understand code when I see it (I can only reverse engineer). I especially understand the KnockKnock Server and MultiClient code on the Sun's Java Networking Tutorial.
Any pointers to which methods I need to learn (I already realize I'm going to need to downcast the SimpleVehicle object back into a SimpleVehicle after it gets sent). Thanks a million.

Answer 1

Ok, try this.
The simplistic application sends colored balls between instances of itself which can be on any machine on your network.
As a ball reaches the left side of the window it is in, it gets sent to the machine on it's left (if any), likewise if it reaches the right side of the window it gets sent to the machine on it's right. If there is no machine on the side the ball is trying to go, it rebounds within the window it is in.
So, a simple test is to run three instances on the same machine, one is regarded as one the left on as on the right and one as in the middle
To start the one on the left:
java Ziniman1 8000 -right localhost:8001
To start the one in the middle:
java Ziniman1 8001 -left localhost:8000 -right localhost:8002
To start the one on the right:
java Ziniman1 8002 -left localhost:8001
Note: You need to start these pretty quickly one after the other or the socket creation will time out. I'll leave that to you to fix ;)
Once the apps are running, move them so they are next to each other, left to right (the port numbers used as the server port are in the JFrame title).
To get create a ball, just click on one of the white panels, use the radio buttons at the bottom to change size/color/direction of the new balls.
Once you've got the hang of the thing, try moving some instances to other machnes.
Enjoy!
import java.io.*;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;
public class Ziniman1
     static final String LEFT= "Left";
     static final String RIGHT= "Right";
     static class Ball
          implements Serializable
          public int x= -1;
          public int y;
          public int size= 10;
          private Color mColor;
          public Ball(Color color) { mColor= color; }
          public Ball(Color color, int y) { mColor= color; this.y= y; }
          public Color getColor() { return mColor; }
          public String toString() { return "(" +x +"," +y +") x " +size; }
     static interface DirectionalListener
          public void left(Ball ball);
          public void right(Ball ball);
     static class Directional
          private DirectionalListener mListener= null;
          public final void setListener(DirectionalListener listener) {
               mListener= listener;
          protected final void fireLeft(Ball ball)
               if (mListener != null)
                    mListener.left(ball);
          protected final void fireRight(Ball ball)
               if (mListener != null)
                    mListener.right(ball);
     static class Server
          extends Directional
          public Server(final int port)
               new Thread() {
                    public void run() {
                         try {
                              ServerSocket listenSocket= new ServerSocket(port);
                              System.err.println("Server listening on port " +port);
                              while (true)
                                   connect(listenSocket.accept());
                         catch (Exception e) {
                              e.printStackTrace();
               }.start();
          private synchronized void connect(final Socket socket)
               Thread thread= new Thread() {
                    public void run() {
                         try {
                              ObjectInputStream is=
                                   new ObjectInputStream(socket.getInputStream());
                              while (true) {
                                   String side= (String) is.readObject();
                                   Ball ball= (Ball) is.readObject();
                                   if (side.equals(RIGHT))
                                        fireLeft(ball);
                                   else
                                        fireRight(ball);
                         catch (Exception e) {
                              e.printStackTrace();
               thread.setDaemon(true);
               thread.start();
     static class Client
          private ObjectOutputStream mOs;
          private String mSide;
          public Client(String host, int port, String side)
               throws Exception
               mSide= side;
               Socket socket= new Socket(host, port);
               mOs= new ObjectOutputStream(socket.getOutputStream());
               System.err.println(
                    mSide +" client connected to " +host +":" +port);
          private void send(Ball ball)
               try {
                    mOs.writeObject(mSide);
                    mOs.writeObject(ball);
               catch (Exception e) {
                    e.printStackTrace();
     static abstract class BallPanel
          extends JPanel
          public void paint(Graphics g)
               g.setColor(Color.WHITE);
               g.fillRect(0, 0, getSize().width, getSize().height);
               Iterator balls= getBalls();
               while (balls.hasNext()) {
                    Ball ball= (Ball) balls.next();
                    g.setColor(ball.getColor());
                    g.fillOval(ball.x, ball.y, ball.size, ball.size);
          public Dimension getPreferredSize() {
               return new Dimension(300, 240);
          public abstract Iterator getBalls();
     static class Gui
          extends Directional
          private Runnable mUpdater= new Runnable() {
               public void run() { mBallPanel.repaint(); }
          private ArrayList mLeft= new ArrayList();
          private ArrayList mRight= new ArrayList();
          private ArrayList mBalls= new ArrayList();
          private BallPanel mBallPanel= new BallPanel() {
               public Iterator getBalls() {
                    return mBalls.iterator();
          public Gui(String title)
               final JRadioButton red= new JRadioButton("Red");
               final JRadioButton green= new JRadioButton("Green");
               final JRadioButton blue= new JRadioButton("Blue");
               ButtonGroup group= new ButtonGroup();
               group.add(red);
               group.add(blue);
               group.add(green);
               final JRadioButton large= new JRadioButton("Large");
               final JRadioButton small= new JRadioButton("Small");
               group= new ButtonGroup();
               group.add(large);
               group.add(small);
               final JRadioButton left= new JRadioButton("Left");
               final JRadioButton right= new JRadioButton("Right");
               group= new ButtonGroup();
               group.add(left);
               group.add(right);
               red.setSelected(true);
               small.setSelected(true);
               right.setSelected(true);
               mBallPanel.addMouseListener(new MouseAdapter() {
                    public void mousePressed(MouseEvent e) {
                         Ball ball= new Ball(
                              red.isSelected() ? Color. RED :
                              blue.isSelected() ? Color.BLUE : Color.GREEN);
                         ball.x= e.getX();
                         ball.y= e.getY();
                         ball.size= large.isSelected() ? 20 : 10;
                         if (left.isSelected())
                              left(ball);
                         else
                              right(ball);
               JPanel panel= new JPanel(new FlowLayout(FlowLayout.LEFT, 4, 4));
               panel.add(red);
               panel.add(blue);
               panel.add(green);
               panel.add(large);
               panel.add(small);
               panel.add(left);
               panel.add(right);
               JFrame frame= new JFrame(title);
               frame.getContentPane().add(mBallPanel, BorderLayout.CENTER);
               frame.getContentPane().add(panel, BorderLayout.SOUTH);
               frame.pack();
               frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               frame.setVisible(true);
          public synchronized void move(int delta)
               Iterator left= mLeft.iterator();
               while (left.hasNext()) {
                    Ball ball= (Ball) left.next();
                    ball.x -= delta;
                    if (ball.x <= 0) {
                         left.remove();
                         mBalls.remove(ball);
                         fireLeft(ball);
               Iterator right= mRight.iterator();
               while (right.hasNext()) {
                    Ball ball= (Ball) right.next();
                    ball.x += delta;
                    if (ball.x >= (mBallPanel.getSize().width -ball.size)) {
                         right.remove();
                         mBalls.remove(ball);
                         fireRight(ball);
               SwingUtilities.invokeLater(mUpdater);
          public synchronized void left(Ball ball)
               mLeft.add(ball);
               mBalls.add(ball);
               if (ball.x < 0)
                    ball.x= mBallPanel.getSize().width -(ball.size/2);
               SwingUtilities.invokeLater(mUpdater);
          public synchronized void right(Ball ball)
               mRight.add(ball);
               mBalls.add(ball);
               if (ball.x < 0)
                    ball.x= ball.size/2;
               SwingUtilities.invokeLater(mUpdater);
     static class Controller
          public Controller(
               final Gui gui, final Server server,
               final Client left, final Client right)
               gui.setListener(new DirectionalListener() {
                    // Ball reached the left
                    public void left(Ball ball)
                         ball.x= -1;
                         if (left == null)
                              gui.right(ball);
                         else
                              left.send(ball);
                    // Ball reached the right
                    public void right(Ball ball)
                         ball.x= -1;
                         if (right == null)
                              gui.left(ball);
                         else
                              right.send(ball);
               server.setListener(new DirectionalListener() {
                    // Ball came from the left
                    public void left(Ball ball) {
                         gui.right(ball);
                    // Ball came from the right
                    public void right(Ball ball) {
                         gui.left(ball);
               Thread thread= new Thread() {
                    public void run() {
                         while (true) {
                              try { sleep(100); }
                              catch (InterruptedException e) { }
                              gui.move(10);
               thread.setDaemon(true);
               thread.start();
     private static final String USAGE=
          "Usage: java Ziniman1 " +
          "<server port> [-left <host:port>] [-right <host:port>]";
        public static void main(String[] argv)
          throws Exception
          if (argv.length < 1)
               barf();
          int leftPort= -1;
          int rightPort= -1;
          String leftHost= "localhost";
          String rightHost= "localhost";
          int serverPort= Integer.parseInt(argv[0]);
          for (int i= 1; i< argv.length; i += 2) {
               if (argv.equals("-left")) {
                    if (argv.length == i+1)
                         barf();
                    String url= argv[i+1];
                    if (url.indexOf(":") < 0)
                         barf();
                    leftHost= url.substring(0, url.indexOf(":"));
                    leftPort= Integer.parseInt(url.substring(url.indexOf(":")+1));
               else if (argv[i].equals("-right")) {
                    if (argv.length == i+1)
                         barf();
                    String url= argv[i+1];
                    if (url.indexOf(":") < 0)
                         barf();
                    rightHost= url.substring(0, url.indexOf(":"));
                    rightPort= Integer.parseInt(url.substring(url.indexOf(":")+1));
          new Controller(
               new Gui("Balls @ " +serverPort),
               new Server(serverPort),
               leftPort > 0 ? new Client(leftHost, leftPort, LEFT) : null,
               rightPort > 0 ? new Client(rightHost, rightPort, RIGHT) : null);
     private static void barf()
          System.err.println(USAGE);
          System.exit(-1);