Develop Reference

A* pathfinding game system exit issue

I am making a game where a badGuy AI is trying to chase me while I use arrow keys to move away. In my main I have a try catch block for if the player attempts to exit the screen border. For some reason however when I click on start I get the exception which I have a system print out saying "GAME OVER, you exited the map". Without my reCalcPath method in the badguy class this does not happen so it must be an issue within this method.
For this method I have a map array. This array is a 40x40 boolean array of 20x20 pixels/cells/squares which states true if I have clicked on that cell position if previously false, painting a white square and visa versa. Now I thought, check the cell position of the badGuy, then check all of his neighbouring cells, if the state of that cell is false i.e. no cell is painted (which means there is no wall blocking him in this sense), then check for the distance between him and my player. I use a Euclidean distance approach by treating xPlayer-xbadGuy, yPLayer-xBadGuy as the opposite and adjacent sides of a triangle. Using pythagoras I get the hypotenuse. Do this for each neighbouring cell, the one with the smallest hypotenuse means shortest distance. Now this is not working at all as when its called the game crashes. Ignore the move method as that is irrelevant if recalcpath won't work
Main
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.image.*;
import java.io.*;
public class AStarMaze extends JFrame implements Runnable, MouseListener, MouseMotionListener, KeyListener {
// member data
private boolean isInitialised = false;
private BufferStrategy strategy;
private Graphics offscreenBuffer;
public boolean map[][] = new boolean[40][40];
private boolean isGameRunning = false;
private BadGuy badguy;
private Player player;
private int startI, startJ;
private int endI, endJ;
private String pFilePath, bgFilePath;
// constructor
public AStarMaze () {
//Display the window, centred on the screen
Dimension screensize = java.awt.Toolkit.getDefaultToolkit().getScreenSize();
int x = screensize.width/2 - 400;
int y = screensize.height/2 - 400;
setBounds(x, y, 800, 800);
setVisible(true);
this.setTitle("A* Pathfinding Demo");
bgFilePath = "C:\\Users\\brads\\IdeaProjects\\PathfindingAssignment\\src\\badguy.png";
pFilePath = "C:\\Users\\brads\\IdeaProjects\\PathfindingAssignment\\src\\player.png";
// load raster graphics and instantiate game objects
ImageIcon icon = new ImageIcon(bgFilePath);
Image img = icon.getImage();
badguy = new BadGuy(img);
icon = new ImageIcon(pFilePath);
img = icon.getImage();
player = new Player(img);
// create and start our animation thread
Thread t = new Thread(this);
t.start();
// initialise double-buffering
createBufferStrategy(2);
strategy = getBufferStrategy();
offscreenBuffer = strategy.getDrawGraphics();
// register the Jframe itself to receive mouse and keyboard events
addMouseListener(this);
addMouseMotionListener(this);
addKeyListener(this);
// initialise the map state
for (x=0;x<40;x++) {
for (y=0;y<40;y++) {
map[x][y]=false;
}
}
isInitialised = true;
}
public boolean[][] getMap(){
return map;
}
// thread's entry point
public void run() {
long loops=0;
while ( 1==1 ) {
// 1: sleep for 1/5 sec
try {
Thread.sleep(100);
} catch (InterruptedException e) { }
try {
// 2: animate game objects
if (isGameRunning) {
loops++;
player.move(map); // player moves every frame
if (loops % 3 == 0) // badguy moves once every 3 frames
badguy.reCalcPath(map,player.x,player.y);
// badguy.move(map, player.x, player.y);
}
// 3: force an application repaint
this.repaint();
}
catch(IndexOutOfBoundsException e){
System.out.println("GAME OVER, you exited the map");
System.exit(-1);
}
}
}
private void loadMaze() {
String filename = "C:\\Users\\brads\\IdeaProjects\\PathfindingAssignment\\maze.txt";
String textinput = null;
try {
BufferedReader reader = new BufferedReader(new FileReader(filename));
textinput = reader.readLine();
reader.close();
}
catch (IOException e) { }
if (textinput!=null) {
for (int x=0;x<40;x++) {
for (int y=0;y<40;y++) {
map[x][y] = (textinput.charAt(x*40+y)=='1');
}
}
}
}
private void saveMaze() {
// pack maze into a string
String outputtext="";
for (int x=0;x<40;x++) {
for (int y=0;y<40;y++) {
if (map[x][y])
outputtext+="1";
else
outputtext+="0";
}
}
try {
String filename = "C:\\Users\\brads\\IdeaProjects\\PathfindingAssignment\\maze.txt";
BufferedWriter writer = new BufferedWriter(new FileWriter(filename));
writer.write(outputtext);
writer.close();
}
catch (IOException e) { }
}
// mouse events which must be implemented for MouseListener
public void mousePressed(MouseEvent e) {
if (!isGameRunning) {
// was the click on the 'start button'?
int x = e.getX();
int y = e.getY();
if (x>=15 && x<=85 && y>=40 && y<=70) {
isGameRunning=true;
return;
}
// or the 'load' button?
if (x>=315 && x<=385 && y>=40 && y<=70) {
loadMaze();
return;
}
// or the 'save' button?
if (x>=415 && x<=485 && y>=40 && y<=70) {
saveMaze();
return;
}
}
// determine which cell of the gameState array was clicked on
int x = e.getX()/20;
int y = e.getY()/20;
// toggle the state of the cell
map[x][y] = !map[x][y];
// throw an extra repaint, to get immediate visual feedback
this.repaint();
// store mouse position so that each tiny drag doesn't toggle the cell
// (see mouseDragged method below)
prevx=x;
prevy=y;
}
public void mouseReleased(MouseEvent e) {}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mouseClicked(MouseEvent e) {}
//
// mouse events which must be implemented for MouseMotionListener
public void mouseMoved(MouseEvent e) {}
// mouse position on previous mouseDragged event
// must be member variables for lifetime reasons
int prevx=-1, prevy=-1;
public void mouseDragged(MouseEvent e) {
// determine which cell of the gameState array was clicked on
// and make sure it has changed since the last mouseDragged event
int x = e.getX()/20;
int y = e.getY()/20;
if (x!=prevx || y!=prevy) {
// toggle the state of the cell
map[x][y] = !map[x][y];
// throw an extra repaint, to get immediate visual feedback
this.repaint();
// store mouse position so that each tiny drag doesn't toggle the cell
prevx=x;
prevy=y;
}
}
//
// Keyboard events
public void keyPressed(KeyEvent e) {
if (e.getKeyCode()==KeyEvent.VK_LEFT)
player.setXSpeed(-1);
else if (e.getKeyCode()==KeyEvent.VK_RIGHT)
player.setXSpeed(1);
else if (e.getKeyCode()==KeyEvent.VK_UP)
player.setYSpeed(-1);
else if (e.getKeyCode()==KeyEvent.VK_DOWN)
player.setYSpeed(1);
}
public void keyReleased(KeyEvent e) {
if (e.getKeyCode()==KeyEvent.VK_LEFT || e.getKeyCode()==KeyEvent.VK_RIGHT)
player.setXSpeed(0);
else if (e.getKeyCode()==KeyEvent.VK_UP || e.getKeyCode()==KeyEvent.VK_DOWN)
player.setYSpeed(0);
}
public void keyTyped(KeyEvent e) { }
//
// application's paint method
public void paint(Graphics g) {
if (!isInitialised)
return;
g = offscreenBuffer; // draw to offscreen buffer
// clear the canvas with a big black rectangle
g.setColor(Color.BLACK);
g.fillRect(0, 0, 800, 800);
// redraw the map
g.setColor(Color.WHITE);
for (int x=0;x<40;x++) {
for (int y=0;y<40;y++) {
if (map[x][y]) {
g.fillRect(x*20, y*20, 20, 20);
}
}
}
// redraw the player and badguy
// paint the game objects
player.paint(g);
badguy.paint(g);
if (!isGameRunning) {
// game is not running..
// draw a 'start button' as a rectangle with text on top
// also draw 'load' and 'save' buttons
g.setColor(Color.GREEN);
g.fillRect(15, 40, 70, 30);
g.fillRect(315, 40, 70, 30);
g.fillRect(415, 40, 70, 30);
g.setFont(new Font("Times", Font.PLAIN, 24));
g.setColor(Color.BLACK);
g.drawString("Start", 22, 62);
g.drawString("Load", 322, 62);
g.drawString("Save", 422, 62);
}
// flip the buffers
strategy.show();
}
// application entry point
public static void main(String[] args) {
AStarMaze w = new AStarMaze();
}
}
Player Class
import java.awt.Graphics;
import java.awt.Image;
public class Player {
Image myImage;
int x=0,y=0;
int xSpeed=0, ySpeed=0;
public Player( Image i ) {
myImage=i;
x=10;
y=35;
}
public void setXSpeed( int x ) {
xSpeed=x;
}
public void setYSpeed( int y ) {
ySpeed=y;
}
public void move(boolean map[][]) {
int newx=x+xSpeed;
int newy=y+ySpeed;
if (!map[newx][newy]) {
x=newx;
y=newy;
}
}
public void paint(Graphics g) {
g.drawImage(myImage, x*20, y*20, null);
}
}
Bad Guy
import java.awt.Graphics;
import java.awt.Image;
public class BadGuy {
Image myImage;
int x=0,y=0;
int distanceX = 0, distanceY = 0;
boolean hasPath=false;
public BadGuy( Image i ) {
myImage=i;
x = 30;
y = 10;
}
public void reCalcPath(boolean map[][],int targx, int targy) {
// TO DO: calculate A* path to targx,targy, taking account of walls defined in map[][]
int totalDistance = 0;
for (int i = -1; i <= 1; i++) {
for (int j = -1; j <= 1; j++) {
if (!map[(x / 20) + i][(y / 20) + j]) {
if ((((targx - x) ^ 2) + ((targy - y) ^ 2)) <= totalDistance) {
totalDistance = (((targx - x) ^ 2) + ((targy - y) ^ 2));
x = ((x / 20) + i);
y = ((y / 20) + j);
return;
}
}
}
}
System.out.println("Not working");
}
// public void move(boolean map[][],int targx, int targy) {
// if (hasPath) {
// // TO DO: follow A* path, if we have one defined
// }
// else if(map[x / 20][y / 20]) {
// hasPath = false;
// }
// }
public void paint(Graphics g) {
g.drawImage(myImage, x*20, y*20, null);
}
}

Your catch block is triggered by an IndexOutOfBoundsException. One easy way to find more information about your error is to log or output the stack trace. You can do that in your code by adding e.printStackTrace() to your catch block:
catch(IndexOutOfBoundsException e){
System.out.println("GAME OVER, you exited the map");
e.printStackTrace()
System.exit(-1);
}
This should show you the line number that's causing your problem.
Looking at the code in your reCalcPath method, my educated guess is the line causing the problem is this: if (!map[(x / 20) + i][(y / 20) + j]) {. If either (x / 20) + i or (y / 20) + j are less than 0 or greater than your map dimensions, it will throw the IndexOutOfBoundsException that you are seeing. In the case where you start and x=30, y=10, i=-1, and j=-1, that evaluates to map[0][-1].
This is what's known as an edge case. You need to check for the case where your x and y are on the edges of the grid. In that situation the neighboring grid cells are outside of the map and will throw an exception.

Repaint() working only half times

i've got a problem with what should be a simple excercise.
I've been asked to make an applet that prints a green oval (filled) that becomes larger until it hits the borders, than start becoming smaller.
This should go on until you close the applet.
Well, i came out with this code
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JApplet;
public class Disco extends JApplet{
private int x;
private int y;
private int r;
private boolean enlarge;
MakeLarger makeLarger;
MakeSmaller makeSmaller;
public void init() {}
public void start() {
x = getWidth()/2;
y = getHeight()/2;
r = 50;
enlarge = true;
makeLarger = new MakeLarger();
makeLarger.start();
}
public void paint(Graphics g) {
g.setColor(Color.GREEN);
g.fillOval(x - r, y- r, r*2, r*2);
}
public void update() {
if(enlarge) {
makeLarger = new MakeLarger();
makeLarger.start();
} else {
makeSmaller = new MakeSmaller();
makeSmaller.start();
}
}
private class MakeLarger extends Thread {
public void run() {
while(true) {
x = getWidth()/2;
y = getHeight()/2;
if(getWidth() > getHeight()) {
if(r < getHeight()/2) {
r++;
repaint();
try {
sleep(25);
} catch(InterruptedException e) {
e.printStackTrace();
}
} else {
enlarge = false;
update();
Thread.currentThread().interrupt();
return;
}
} else {
if(r < getWidth()/2) {
r++;
repaint();
try {
sleep(25);
} catch(InterruptedException e) {
e.printStackTrace();
}
} else {
enlarge = false;
update();
Thread.currentThread().interrupt();
return;
}
}
}
}
}
private class MakeSmaller extends Thread {
public void run() {
while(true) {
x = getWidth()/2;
y = getHeight()/2;
if(r > 50) {
r--;
repaint();
revalidate();
try {
sleep(25);
} catch(InterruptedException e) {
e.printStackTrace();
}
} else {
enlarge = true;
update();
Thread.currentThread().interrupt();
return;
}
}
}
}
}
When I start my applet the oval start growing correctly until it hits the border and then suddently stop.
The first thing i thought was that it wasn't getting smaller correctly. But a little System.out.println work showed me that all computation was going on correctly, the problem is that the applet repaint only while the makeLarger thread is active, when the makeSmaller thread is at work the call to repaint() doesn't work!
If i resize the applet window while the makeSmaller Thread is at work it repaints correctly showing me the oval getting smaller.
Can, please, someone enlight me on this odd behavior?
What am I missing?
thank you everyone for your most appreciated help and sorry if my english is so poor!

I can't say that I've looked at all the code, but a couple of suggestions:
Paint in a JPanel's paintComponent override.
This is key: call the super.paintComponent method in the override, first line. This gets rid of prior images so that the image can get smaller.
Display the JPanel within your JApplet by adding it to the applet.
Myself, I'd use a single Swing Timer for the animation loop, and would use a boolean to decide which direction the sizing should go.
But regardless of whether I were using a Swing Timer or a Runnable placed in a Thread, I'd try to keep things a simple as possible, and that would mean using a single Timer that changes direction of resizing or a single Runnable/Thread that changes size of resizing based on a boolean. This swapping of threads that you're doing just serves to overly and unnecessarily complicate things, and could quite possibly be the source of your error.
As a side recommendation, you almost never want to extend Thread. Much better is to create classes that implement Runnable, and then when you need to run them in a background thread, create a Thread, passing in your Runnable into the Thread's constructor, and then calling start() on the Thread.

First things first, thank you all for helping me!
The problem I was trying to solve was this:
Make an applet which shows a green oval getting larger until it hits the borders, then it should start getting smaller up to a fixed radius (50), then it should start over (and over) again. Solve the problem by making two threads (by extending Thread).
Just to explain why the algorithm was so odd!
Well, reading your suggestions I fixed my code, now it's running properly. Here is the code if anyone will need this.
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import javax.swing.JApplet;
import javax.swing.Timer;
public class Disco extends JApplet{
private int x;
private int y;
private int r;
private boolean enlarge;
MakeLarger makeLarger;
MakeSmaller makeSmaller;
public void init() {}
public void start() {
x = getWidth()/2;
y = getHeight()/2;
r = 50;
enlarge = true;
makeLarger = new MakeLarger();
makeLarger.start();
Timer timer = new Timer(1000/60, new ActionListener() {
public void actionPerformed(ActionEvent e) {
repaint();
}
});
timer.start();
}
public void paint(Graphics g) {
BufferedImage offScreenImage = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_4BYTE_ABGR_PRE);
Graphics2D g2D = (Graphics2D) offScreenImage.getGraphics();
g2D.clearRect(0, 0, getWidth(), getHeight());
g2D.setColor(Color.GREEN);
g2D.fillOval(x - r, y- r, r*2, r*2);
g.drawImage(offScreenImage, 0, 0, this);
}
public void update() {
if(enlarge) {
makeLarger = new MakeLarger();
makeLarger.start();
} else {
makeSmaller = new MakeSmaller();
makeSmaller.start();
}
}
private class MakeLarger extends Thread {
public void run() {
while(true) {
x = getWidth()/2;
y = getHeight()/2;
if(getWidth() > getHeight()) {
if(r < getHeight()/2) {
r++;
try {
sleep(25);
} catch(InterruptedException e) {
e.printStackTrace();
}
} else {
enlarge = false;
update();
Thread.currentThread().interrupt();
return;
}
} else {
if(r < getWidth()/2) {
r++;
try {
sleep(25);
} catch(InterruptedException e) {
e.printStackTrace();
}
} else {
enlarge = false;
update();
Thread.currentThread().interrupt();
return;
}
}
}
}
}
private class MakeSmaller extends Thread {
public void run() {
while(true) {
x = getWidth()/2;
y = getHeight()/2;
if(r > 50) {
r--;
try {
sleep(25);
} catch(InterruptedException e) {
e.printStackTrace();
}
} else {
enlarge = true;
update();
Thread.currentThread().interrupt();
return;
}
}
}
}
}

How to draw an image over another?

So I am working on a project for a class. And I've been struggling with this issue for a while now. The code below is a start screen, and when the enter key is pressed (when atTitle turns to false) I would like it to draw the next image. The problem with that is I can not think of a way for it to draw the next image when it turns to false. I've tried using ifs and whiles. Mainly the problem is that you obviously put another public void paintComponent in an if statement. And I can't carry the Graphics g variable to the KeyPressed method.
I'm stuck.
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
AffineTransform at = new AffineTransform();
g2.setTransform(at);
if (atTitle == true) {
g.drawImage(titlescreen, 0, 0, this);
if (start_visible == true) {
g.drawImage(start_symbol, -70, 30, this);
jf.addKeyListener(this);
}
}
}
public void keyTyped(KeyEvent e) {}
public void keyReleased(KeyEvent e) {}
public void keyPressed(KeyEvent e) {
//int keyCode = e.getKeyCode();
if (atTitle == true) {
if (e.getKeyCode() == KeyEvent.VK_ENTER) {
atTitle = false;
System.out.println("It Works.");
}
}
}

you need to call repaint and wait system calls to paint function. you should not keep graphic object and use it in a function is not child function of paint
as a simple canvas component, you can use this one
class Game extends Canvas implements Runnable {
private Thread thread;
public Game(){
thread = new Thread(this);
thread.start();
}
#Override
public void paint(Graphics g) {
// paint your game
}
public void stop(){
thread = null;
}
#Override
public void run() {
while (thread == Thread.currentThread()){
long ti = System.currentTimeMillis();
repaint();
long ti2 = System.currentTimeMillis();
long waitTime = 60 - (ti2-ti);
if (waitTime > 0){
try {
Thread.sleep(waitTime);
} catch (Exception e){
}
}
}
}
}

If you are using JPanel you should:
panel.setFocusable(true);
panel.requestFocusInWindow();
Referenced here
Also you can call panel.revalidate() and/or panel.repaint() in the key press.
Maybe try to stay away from JPanel and Swing altogether (not suited for gaming, but for forms) and just use the graphics2D functionality with a Window, Frame and Canvas, as suggested in previous comment.

Java can you make a predefined class in java serializable?

I want to send a Image object over a network in java.
Im getting this error.
java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException: sun.awt.image.OffScreenImage
The java Image object doesn't implement Serializable. Is there a way to get around this?
Ive already tried making a subclass of image and implement it but Then I got errors when using the createImage method. Thanks for any help.
EDIT*
Ok here is the code but there is kinda a lot. The idea of the program is for it to be a pictionary game. Someone can draw using basic tools and it will send it over a network and draw that image on other clients screen.
This is my basic draw area where the user and draw using a line tool. On mouse Released it will try and send the Image object over to the server.
class PadDraw extends JComponent {
Image image;
Graphics2D graphics2D;
int currentX, currentY, oldX, oldY;
int lineSize = 1;
public PadDraw() {
setDoubleBuffered(false);
addMouseListener(new MouseAdapter() {
#Override
public void mousePressed(MouseEvent e) {
oldX = e.getX();
oldY = e.getY();
}
});
addMouseMotionListener(new MouseMotionAdapter() {
#Override
public void mouseDragged(MouseEvent e) {
currentX = e.getX();
currentY = e.getY();
//graphics2D.drawLine(oldX, oldY, currentX, currentY); //this is where it does the drawing
//It seems to draw a line between the old coordinate point and the new coordinate point rather than drawing it as points
//Test to see if I can get a drawoval to work rather than a line
//graphics2D.fillOval(currentX-1, currentY-1, 2, 2);
//if this works it should draw an oval at the cursor position rather than drawing a line
//technically it works, but without a line it causes gaps
//I may have found it. Testing the setStroke method
graphics2D.setStroke(new BasicStroke(lineSize));
graphics2D.drawLine(oldX, oldY, currentX, currentY);
repaint();
oldX = currentX;
oldY = currentY;
}
});
addMouseListener(new MouseAdapter() {
#Override
public void mouseReleased(MouseEvent e) {
try {
clientOutputStream.writeObject(image);
} catch (IOException ex) {
Logger.getLogger(ChatClient.class.getName()).log(Level.SEVERE, null, ex);
}
}
});
}
#Override
public void paintComponent(Graphics g) {
if (image == null) {
image = (Image) createImage(getSize().width, getSize().height);
graphics2D = (Graphics2D) image.getGraphics();
graphics2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
clear();
}
g.drawImage(image, 0, 0, null);
}
public void updateImage(Image image){
this.image = image;
repaint();
}
public void clear() {
graphics2D.setPaint(Color.white);
graphics2D.fillRect(0, 0, getSize().width, getSize().height);
//graphics2D.setPaint(Color.BLACK);
lineSize = 1;
repaint();
}
public void fill(){
Color c = findColor();
graphics2D.setPaint(c);
graphics2D.fillRect(0, 0, getSize().width, getSize().height);
repaint();
}
public void changeColor(Color theColor) {
graphics2D.setPaint(theColor);
repaint();
}
public Color findColor() {
return graphics2D.getColor();
}
public void changeSize(int size) {
lineSize = size;
}
}
This is my threaded class for the image on the server.
private static class Handler2 extends Thread {
private Socket socket1;
private ObjectInputStream serverInputStream;
private ObjectOutputStream serverOutputStream;
public Handler2(Socket sock1) {
socket1 = sock1;
}
#Override
public void run() {
Image image = null;
try {
serverInputStream = new ObjectInputStream(socket1.getInputStream());
serverOutputStream = new ObjectOutputStream(socket1.getOutputStream());
oos.add(serverOutputStream);
while (true) {
image = (Image)serverInputStream.readObject();
for (ObjectOutputStream ooss : oos) {
ooss.writeObject(image);
}
}
} catch (IOException e) {
System.out.println(e);
} catch (ClassNotFoundException ex) {
Logger.getLogger(ChatServer.class.getName()).log(Level.SEVERE, null, ex);
} finally {
if (serverOutputStream != null) {
oos.remove(serverOutputStream);
}
try {
socket1.close();
} catch (IOException e) {
System.out.println(e);
}
}
}
}
And back in the client I have a method for getting the image back from the server.
public void run2() throws IOException, ClassNotFoundException, InterruptedException {
// Make connection and initialize streams
serverAddress = getServerAddress();
Socket socket2 = new Socket(serverAddress, 9999);
//String theIP = getIP();
//Socket socket2 = new Socket(theIP, 9999);
// Process all messages from server, according to the protocol.
clientOutputStream = new ObjectOutputStream(socket2.getOutputStream());
clientInputStream = new ObjectInputStream(socket2.getInputStream());
while (true) {
Image ni = (Image)clientInputStream.readObject();
drawPad.updateImage(ni);
}
}
I know my code is kinda bad. I split thinks up a lot to test individual parts. As fare as the network code. It should work. The only problem I believe is that its not serializable.

The java Image object doesn't implement Serializable. Is there a way to get around this?
You can Serialize it yourself. You can wrap it with a class which is Externalizable, or you can write the data in the image without using writeObject.

Simple answer: no. sun.awt.image.OffScreenImage is not meant to be directly serialized so you can't just add the Serializable interface on a child class to make it serializable.
You have to find a work around, I'm not sure what is this image about but, for example, if it's from a set of known images then you can just send a key to it over the network so that you can recover it on the otherside. If you need to pass directly the image data then you'll have to encapsulate it in another object and rebuild the OffScreenImage on the other side.
You can extend the class and make it serializable with some work but since it seems to be a class tighly coupled with the operating system it will need some thinking.

How to put two different tasks in Threads

I have following code. in this code i have an image which moves from left to right and a button which has an event. but i want to put these both tasks in Threads. so that it can work properly. the problem with this code is that button event does not work until it reaches to the right most point.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
class MyImage extends JFrame implements ActionListener
{
static int xPixel = 20;
Image myImage, offScreenImage;
Graphics offScreenGraphics;
JPanel p = new JPanel();
Button btn = new Button("bun");
JFrame f = new JFrame();
public MyImage()
{
myImage = Toolkit.getDefaultToolkit().getImage("mywineshoplogo.jpg");
setExtendedState(JFrame.MAXIMIZED_BOTH);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);
add(p);
p.add(btn);
moveImage();
btn.addActionListener(this);
}
public void update(Graphics g)
{
paint(g);
}
public void paint(Graphics g)
{
int width = getWidth();
int height = getHeight();
if (offScreenImage == null)
{
offScreenImage = createImage(width, height);
offScreenGraphics = offScreenImage.getGraphics();
}
// clear the off screen image
offScreenGraphics.clearRect(0, 0, width + 1, height + 1);
// draw your image off screen
offScreenGraphics.drawImage(myImage, xPixel, 10, this);
// draw your image off screen
// show the off screen image
g.drawImage(offScreenImage, 0, 0, this);
// show the off screen image
}
void moveImage() //left to right move
{
Thread hilo = new Thread() {
public void run() {
try {
for (int i = 0; i < 530; i++)
{
xPixel += 1;
repaint();
// then sleep for a bit for your animation
try
{
Thread.sleep(4);
} /* this will pause for 50 milliseconds */
catch (InterruptedException e)
{
System.err.println("sleep exception");
}
}
} //try
catch (Exception ex) {
// do something...
}
}
};
hilo.start();
}
/* void moveimg() // right to left move
{
for (int i = 529; i > 0; i--)
{
if (i == 1)
{
moveImage();
}
xPixel -= 1;
repaint();
// then sleep for a bit for your animation
try
{
Thread.sleep(40);
} // this will pause for 50 milliseconds
catch (InterruptedException e)
{
System.err.println("sleep exception");
}
}
} */
public void actionPerformed(ActionEvent ae)
{
try
{
if (ae.getSource() == btn)
{
p.setBackground(Color.RED);
}
}
catch (Exception e)
{
System.out.println("error");
}
}
public static void main(String args[])
{
MyImage me = new MyImage();
}
}

Whenever you are about to write Thread.sleep in your GUI code, stop yourself and introduce a task scheduled on Swing's Timer. This is exactly what you need with your code: schedule each update as a separate scheduled task on Timer. Timer is quite simple and straightforward to use, see for example this official Oracle tutorial.