Java JApplet doesn't remove an old rectangle once it is moved

Question

I am new to graphics and japplets, and I made a rectangle that goes across the screen. But for some reason, it just draws a line across, not removing the old instance of the rectangle once it moves.

Main:

public class Main extends JApplet implements Runnable {
    private static final long serialVersionUID = 1L;
    private static int width = 900;
    private static int height = 600;
    public static int fps = 60;
    public Thread thread = new Thread(this);
    public static Ailoid ailoid = new Ailoid();

    public void init() {
        setSize(width, height);
        setBackground(Color.white);

        ailoid.setLocation(new Location(100, 100));
        AlienManager.registerAlien(ailoid);
    }

    public void paint(Graphics g) {
        g.setColor(Color.green);
        for (Alien alien : AlienManager.getAliens()) {
            Location loc = alien.getLocation();
            int x = loc.getX();
            int y = loc.getY();
            g.fillRect(x, y, 10, 20);
        }
    }

    // Thread start
    @Override
    public void start() {
        thread.start();
    }
    // Thread stop
    @SuppressWarnings("deprecation")
    @Override
    public void destroy() {
        thread.stop();
    }

    @Override
    public void run() {
        while (true) {
            Updater.run();
            repaint();
            try {
                // 1000 divided by fps to get frames per millisecond
                Thread.sleep(1000 / fps);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

Updater:

public class Updater {
    public static void run() {
        for (Alien alien : AlienManager.getAliens()) {
            Location loc = alien.getLocation();
            int x = loc.getX();
            int y = loc.getY();
            alien.setLocation(new Location(x, y));
        }
    }
}

Why doesn't it remove the old graphics? thank you!

Answer 1

Your main problem is that your paint(...) method does not call the super method, the method that allows the component to redraw its contents:

public void paint(Graphics g) {
    super.paint(g);
    //....

Having said that, you're far better off not drawing in a top level window but rather in the paintComponent method of a JPanel that is displayed by the applet. If you do this correction, then do the same thing -- call the super method.

class MyPanel extends JPanel {

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        //....

---

As an aside, this code:

public class Updater {
    public static void run() {
        for (Alien alien : AlienManager.getAliens()) {
            Location loc = alien.getLocation();
            int x = loc.getX();
            int y = loc.getY();
            alien.setLocation(new Location(x, y));
        }
    }
}

Doesn't look like it's moving things all that much. In fact per this code, your aliens should stay completely still.

---

Edit
Also, this is never code, code you should never have in your application:

@SuppressWarnings("deprecation")
@Override
public void destroy() {
    thread.stop();
}

There's a reason that thread's stop() method is deprecated and should never be called. If you're curious as to why, please check out the API.