How to paint a JFrame at creation?

Question

I'm trying to paint a background image onto a JFrame for a life simulation game I'm making. This is my code:

public class MainFrame extends JFrame  {
 //creates image variables
Image background;

 public MainFrame(int w, int h) {
    //creates new JFrame and sets some other properties
    super("Life Simulation");
    setLayout(new FlowLayout());
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(new Dimension(w,h));
    //creates images
    background = Toolkit.getDefaultToolkit().createImage("img/default.jpg");
    this.repaint();
    setVisible(true);
}
 @Override
 public void paint(Graphics g) {
         super.paint(g);
         Graphics2D g2d = (Graphics2D) g;
         g.drawImage(background,0,0,null);
     }
}

I've tried to repaint it before setting it visible, but nothing. When I launch my program from my main method, the JFrame is simply blank. However, if I resize it in the slightest, the paint method is called and the background image is painted.

This is my main method:

public class Main {

public static void main(String[] args) {
    MainFrame frame = new MainFrame(1080,720);
    frame.repaint(); //tried invoking paint() here as well but again to no avail
}

}

EDIT: I believe it is also worth mentioning that I have little to no experience beforehand with using paint() or any of its variants, only knowledge of how it SHOULD be implemented and its abilities.

Answer 1

Oh, I'd like to stress the importance of providing an appropriate ImageObserver object when calling the drawImage method. Instead of passing null , I'd recommend passing this :

g.drawImage(background, 0, 0, this);

You can read about the importance of specifying an ImageObserver when loading images asynchronously via Toolkit.createImage .

Aside: Though paintComponent isn't defined for JFrame, I would recommend avoiding overriding JFrame's paint method. Instead, you could create either a reusable ImagePanel class, or just use an anonymous class, and then use that to set the content pane of your JFrame.

Here's an example of an ImagePanel class:

class ImagePanel extends JPanel {
    private Image image;

    public ImagePanel(Image image) {
        this.image = image;
    }

    @Override protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(image, 0, 0, this);
    }
}

Or if you prefer an anonymous class:

setContentPane(new JPanel() {
    @Override protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(image, 0, 0, this);
    }
});