paintComponent isnt called

Question

In JFrame i have some buttons and BufferedImgae which will be used to draw on it some shapes and pictures using my own methods ( drawLine, drawing raster picture pixel by pisel and so on). Here is how i add things to JFrame

public class Main extends javax.swing.JPanel {

    JPanel panel;
    JFrame fr;

    Graphics2D g2;
    ImageIcon icon;
    BufferedImage img;

    public void init()
    {
        fr = new JFrame("Lab 2");
        fr.setMinimumSize(new Dimension(1350, 650));
        fr.setMaximumSize(fr.getMinimumSize());
        fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel glowny = new JPanel(new BorderLayout());
        glowny.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
        glowny.add(getBorderCenter(), BorderLayout.CENTER); 
        fr.add(glowny);
        fr.pack();
        fr.setVisible(true);
    }

    private JScrollPane getBorderCenter()
    {
        img = new BufferedImage( fr.getWidth()-40, fr.getHeight()-40-50, BufferedImage.TYPE_INT_RGB); //20+20 odstępy w glowny, 50 - szerokość paska z guzikami
        icon = new ImageIcon(img); 
        return new JScrollPane(new JLabel(icon));
    }

    public static void main(String [] args)
    {
        Main m = new Main();
        m.init();
    }
}

Then i try to draw on BufferedImage using double buffering. The following example change BufferedImage color from black (current) to white.

@Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        System.out.println("paintComponent");

        WritableRaster raster = img.getRaster();
        DataBuffer db = raster.getDataBuffer();

        int[] pixels = ((DataBufferInt)db).getData();        
        int adres = 0;
        for (int y = 0; y < img.getHeight(); y++) 
        {
            adres = y * img.getWidth();
            for (int x = 0; x < img.getWidth(); x++) 
            {
                pixels[adres] = 16777215;
                adres += 1;
            }
        }

        Graphics2D g2dComponent = (Graphics2D) g;
        g2dComponent.drawImage(img, null, 0, 0); // draw buffer on screen
    }

As i understand repaint() force calling paintComponent(). The problem is that it doesnt matter how i call repaint()

fr.repaint();
glowny.repaint();
repaint();

its never called.

Answer 1

You never add Main to the GUI. You need to add the JPanel that overrides the paintComponent, the this, to the GUI somewhere and you don't. For example, you would need somewhere something like:

someComponentShownInGui.add(this);

I would get rid of the glowny variable and instead use this.

i.e.,

public void init()
{
    fr = new JFrame("Lab 2");
    fr.setMinimumSize(new Dimension(1350, 650));
    fr.setMaximumSize(fr.getMinimumSize());
    fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    // JPanel glowny = new JPanel(new BorderLayout());
    // glowny.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
    // glowny.add(getBorderCenter(), BorderLayout.CENTER); 
    // fr.add(glowny);

    setLayout(new BorderLayout();
    setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
    add(getBorderCenter(), BorderLayout.CENTER); 
    fr.add(this);

    fr.pack();
    fr.setVisible(true);
}

Note that anything components added to the drawing JPanel will cover up its image, and so you may need to make some non-opaque.

Answer 2

You should add all you components inside a JPanel then override paintComponent method of the panel. (JFrame doesn't have paintComponet but paint method and it's not recommanded to override it)