BackGround in my JPanel, how can I resolve this?

Question

Well my problem is that I'm making the stars move with a thread, they move verticaly and it works good but i do a random X for the star and sometimes it intersecs other stars like this :

This is my code for the JPanel:

class Backgroundmoving

public class Backgroundmoving extends JPanel {
    ArrayList<starmoving> star;

    public Backgroundmoving() {
        this.setSize(650, 501);
        star = new ArrayList<>();
        for (int i = 0; i < 20; i++)
            this.addStar();
    }

    public void addStar() {
        int x, y;

        x = (int) (Math.random() * 625);

        y = (int) (Math.random() * 476);

        starmoving e = new starmoving(x, y);
        star.add(e);
        Thread t = new Thread(e);
        t.start();
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        draw(g);
    }

    public void draw(Graphics g) {
        g.drawImage(new ImageIcon("background.png").getImage(), 0, 0, 650, 501, null);
        for (int i = 0; i < star.size(); i++) {
            star.get(i).draw(g);
        }
        repaint();
    }

    public static void main(String[] args) {
        // TODO code application logic here
        JFrame gui = new JFrame();
        gui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        gui.setSize(650, 510);
        gui.setResizable(false);
        gui.add(new Backgroundmoving());

        gui.setVisible(true);
    }

}

class starmoving

public class starmoving implements Runnable {

    int x;
    int y;
    int yVel;

    public starmoving(int x, int y) {
        this.x = x;
        this.y = y;
        yVel = 1;
    }

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

    private void move() {
        y += yVel;
        if (y > 476) {
            y = 0;
            x = (int) (Math.random() * 625);
        }
    }

    private boolean isOffScreen() {

        if (y <= 476)
            return false;
        return true;
    }

    public void draw(Graphics g) {
        g.drawImage(new ImageIcon("star.png").getImage(), x, y, 12, 12, null);
    }

    @Override
    public void run() {
        while (true) {
            move();

            try {
                Thread.sleep(7);
            } catch (InterruptedException ex) {
                System.out.println(ex.getMessage());
            }

        }
    }

}

Answer 1

i don't want stars intersecting, was thinking in a if before the random X but how can i know if other star is in that X ?

You have an ArrayList that contains all your "StarMoving" objects. So you need to iterate through that list to make sure that none of the object intersect.

On top of that you have other problems.

1. Use proper Java class names. Java classes SHOULD start with an upper case character. (ie. "starmoving" is wrong)

2. Don't use multiple Threads for the animation. You current code starts 20 Threads. You should have a single Thread and then iterate through your ArrayList to move all the stars.

3. Don't read the image in the draw() method. Currently you code is reading the image every 7ms. This is not very efficient. The image should be read once and then stored as a property of your class.