Synchronising two different objects

Question

Hi everyone,
I am asking for your help while i am writing an example code. I want to learn how to synchronise two differents objects.
The first class is actually a MIDlet, which has to increase a counter and display it until the second class send a message to the midlet. At this moment, the midlet has to display the message. I am actualy thinking about adding another thread, but lets speak only about the midlet and the calculator :)
I think there is a problem in my way of thinking, that's why i am asking for your help :
Here is an extract of my midlet code :
public class Main extends MIDlet implements CommandListener {
    private static boolean isFromCalculator    = false;
    private static boolean displayUsed = false;
    Command exitCmd = new Command("Exit", Command.EXIT, 1);
    Command pauseCmd = new Command("Pause", Command.OK, 1);
    TextBox tb = new TextBox("Principal", "", 200, 1);
    int cpt = 0;
    Calculator calc = new Calculator();
    public Main()
        calc.start();
        tb.addCommand(exitCmd);
        tb.addCommand(pauseCmd);
        tb.setCommandListener(this);
    public void startApp() {
        process();
    public void pauseApp() {
    public void destroyApp(boolean unconditional) {
    public void process()
        // Tant que l'on a pas d'infos des deux autres threads, on compte !
        while ( !isFromCalculator )
            if ( displayUsed )
                synchronized(this){
                    try{
                        wait();
                    catch (Exception e)
                        e.printStackTrace();
            display("Principal : " + cpt + "\n", 1);
            cpt++;
        deroute();
    public void display(String message, int time)
        displayUsed = true;
        tb.delete(0, tb.size());
        tb.insert(message, tb.size());
        Display.getDisplay(this).setCurrent(tb);
        synchronized(this)
            try{
                wait(time);
            catch (Exception e )
            {e.printStackTrace();}
        displayUsed = false;
        notifyAll();
    public void deroute()
        if ( isFromCalculator == true )
            // Manage Calculator message
            display(calc.Message(), 1000);
            isFromCalculator = false;
        process();
    public static void setBoolFromCalculator(boolean b)
        isFromCalculator = b;
    public static boolean isDisplayUsed()
        return displayUsed;
}And here is my calculator code :
public class Calculator extends Thread{
    private int iterator;
    private boolean quit;
    public Calculator()
        iterator = 0;
        quit = false;
    public void run()
        while ( !quit )
            iterator++;
            // 100 Boucles maxi
            if ( iterator > 100)
                quit = true;
            try{
                sleep(1000);
            catch (InterruptedException e )
                System.out.println("Calculator sleep error : ");
                e.printStackTrace();
            if (iterator % 10 == 0)
                if ( !Main.isDisplayUsed() )
                    synchronized(this)
                        try{
                            wait();
                        catch(Exception e)
                            e.printStackTrace();
                Main.setBoolFromCalculator(true);
    public String Message()
        return " Calculator - Iterator = " + iterator +"\n" ;
}May i ask your any advice ?
Regards,

Answer 1

t00f wrote:
Hi everyone,
I am asking for your help while i am writing an example code. I want to learn how to synchronise two differents objects.Note that you don't "synchronize objects." You synchronize methods or blocks of code on objects (or, more properly I suppose, on their locks). This may seem like a petty distinction, but when you say "synchronize objects," it sounds like you think that you're controlling access to an object. That is not the case. If you need further explanation, let me know.