How to draw a gif in another thread in java?

Question

This is my first time trying to use another thread in java, could someone tell me how to make it work please? I've read others topics about it, but I didn't find a solution.

I'd like to draw a gif in another thread (drawn at at a random position and during the duration of its animation).

The problem is that drawImage() in the second thread just doesn't do anything. My counter works well (it prints 1.. 2.. 3 ...), but no image is drawn (or I can't see it).

The condition is false at the begining, then it is true at one moment (to create only one new thread and no more), then it is false again.

if (condition) {
    (new ThreadGif(this,g)).start();
}

However when I remove the condition in paintComponent(), it draws something which means that drawImage() works. So when it creates lots of new threads, every image of the gif is drawn at a random location and it starts the gif again and again (and the counter(s) still work well).

This could be fine, but I don't think creating thousands of new thread is the answer : I just need one. And also, I need just one random position for each gif, not one different for each image of the gif.

I hope I've been clear enough. Please help me understand how to make it work :) Thank you very much.

Here are simplified versions of my two classes :

ThreadGif.java :

public class ThreadGif extends Thread { 

    Screen screen;
    Graphics g;
    boolean running = true;

    public ThreadGif(Screen screen, Graphics g) {
        this.g = g;
        this.screen = screen;
    }

    public void run() {

        int aleaX = new Random().nextInt(300)/100;
        int aleaY = new Random().nextInt(300)/100;
        int compt = 1;

        while (running) {
            g.drawImage(new ImageIcon("res/feu.gif").getImage(), screen.tailleCase*aleaX, screen.tailleCase*aleaY, screen.tailleCase*2, screen.tailleCase*2, screen);
            System.out.println("thread " + compt);
            compt++;
            try {
                Thread.sleep(sleepTime);
            } catch(InterruptedException e) {
                e.printStackTrace ();
            }
        }
    }       
}

Screen.java :

public class Screen extends JPanel implements Runnable {
    Thread thread = new Thread(this);

    public Screen(Frame frame) {
        this.frame = frame;
        thread.start();
    }

    public void paintComponent(Graphics g) {
        g.clearRect(0, 0, this.frame.getWidth(), this.frame.getHeight());
        if (condition) {
            (new ThreadGif(this,g)).start();
        }
    }

    public void run() { 
        while (running) {
            repaint();
            try {
                Thread.sleep(sleepTime);
            } catch(InterruptedException e) {
                e.printStackTrace ();
            }
        }
        System.exit(0);
    }
}

Answer 1

Pretty simple: you mixed two approaches up.
The paintComponent-method launches a new ThreadGif every time it's called and ThreadGif itself paints within it's thread until it's terminated, but without refreshing the screen.

These two approaches combined might either result in strange behaviour, e.g. two images painted where pieces overlay each other, or the new ThreadGif simply renders the new image every-time you repaint the screen.

Solution: Start by assigning each class specific tasks, without mixing anything up, or splitting tasks between two classes. E.g.:

• ThreadGif doesn't paint anything itself, but repaints the Screen. The Screen can request the image that should be displayed from ThreadGif.
• Make ThreadGif an own Component that handles it's own rendering and ommit the Screen-class from painting anything.