J2me synchronization of methods - java-me

Question

I initializing a variable in my startApp method and after I do so I call a thread to check the whole run time of the app what is the current screen (canvas) and activating it's thread
import java.io.IOException;
import java.io.InputStream;
import zuma.core.MyGameCanvas;
import zuma.core.Game;
import zuma.gui.LevelSelectionMenu;
import zuma.gui.MainMenu;
import javax.microedition.lcdui.Display;
import javax.microedition.media.Manager;
import javax.microedition.media.MediaException;
import javax.microedition.midlet.*;
import zuma.util.AudioManager;
public class Midlet extends MIDlet implements Runnable{
private static Display display;
private static Game game;
private static MainMenu mainMenu;
private static LevelSelectionMenu levelSelectionMenu;
private static MyGameCanvas myGameCanvas;
private Thread t;
/**
* Specifies what happens when starting the application.
*/
public void startApp() {
display = Display.getDisplay(this);
display.setCurrent(getMainMenu());
t = new Thread(this);
t.start();
}
/**
* Specifies what happens when pausing the application.
*/
public void pauseApp() {
}
/**
* Specifies what happens when exiting the application.
* #param unconditional
*/
public void destroyApp(boolean unconditional) {
//game.getLevelsRecordStore().closeRecordStore();
notifyDestroyed();
}
/**
*
* #return the display.
*/
public static Display getDisplay() {
return display;
}
/**
*
* #return the game.
*/
public static Game getGame() {
if (game == null) { //|| game.getLevels() == null) {
game = new Game();
}
return game;
}
/**
*
* #return the mainMenu.
*/
public static synchronized MainMenu getMainMenu() {
if (mainMenu == null) {
mainMenu = new MainMenu();
}
return mainMenu;
}
/**
*
* #return the levelSelectionMenu.
*/
public static LevelSelectionMenu getLevelSelectionMenu() {
if (levelSelectionMenu == null) {
levelSelectionMenu = new LevelSelectionMenu(getGame());
}
return levelSelectionMenu;
}
/**
*
* #return the myGameCanvas.
*/
public static MyGameCanvas getMyGameCanvas() {
if (myGameCanvas == null) {
myGameCanvas = new MyGameCanvas();
}
return myGameCanvas;
}
/**
* Starts the thread of the current display.
*/
public synchronized void run() {
while (true) {
if (display.getCurrent().equals(mainMenu)) {
if (mainMenu.getT() == null || !mainMenu.getT().isAlive()) {
mainMenu.init();
}
if (getMainMenu().isQuitRequest()) {
destroyApp(true);
}
}
else if (display.getCurrent().equals(levelSelectionMenu)) {
if (levelSelectionMenu.getT() == null || !levelSelectionMenu.getT().isAlive()) {
levelSelectionMenu.init();
}
}
else if (display.getCurrent().equals(myGameCanvas)) {
if (myGameCanvas.getT() == null || !myGameCanvas.getT().isAlive()) {
myGameCanvas.init(game.getLevels()[game.getChosenLevel()]);
}
}
try {
Thread.sleep(10);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
}
}
Now as you can see I first init the variable mainMenu in this line
display.setCurrent(getMainMenu());
and only then call run method, but somehow SOMETIMES it gives me null pointer exception on this line
if (display.getCurrent().equals(mainMenu))
so somehow the thread is getting called even before the mainMenu is being created and I have no idea how, what I tried is making the run method and the getMainMenu methos synchronized but it didnt work, can anyone help me ?

Answer 1

The thread is not getting called before the main menu is created. After setCurrent is called, there is actually a delay before the displayable is made visible. The doc says: "The setCurrent() method returns immediately, without waiting for the change to take place. Because of this delay, a call to getCurrent() shortly after a call to setCurrent() is unlikely to return the value passed to setCurrent()."