Is it possible to write paint on screen program in java/swing?

Question

I'm writing paint on screen program using Java Swing. It working on ubuntu linux. But windows shows black screen instead of transparent panel. I included similar example code. What is wrong in my code?

import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

public class Example {

public static final Color COLOR_TRANSPARENT = new Color(0,0,0,0);

public Example() {
    Canvas drawArea = new Canvas();
    drawArea.setBackground(COLOR_TRANSPARENT);
    drawArea.setOpaque(true);

    JWindow drawingFrame = new JWindow();
    drawingFrame.setBackground(COLOR_TRANSPARENT);
    drawingFrame.setContentPane(drawArea);
    drawingFrame.pack();
    drawingFrame.setSize(640, 460);
    drawingFrame.setVisible(true);
    drawingFrame.setLocationRelativeTo(null);
    drawingFrame.setAlwaysOnTop(true);
}

public static void main(String[] args){
    SwingUtilities.invokeLater(Example::new);
}

class Canvas extends JPanel{

    private Image image;
    private Graphics2D g2;

    public Canvas() {
        super();
        addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                int x = e.getX();
                int y = e.getY();
                g2.setPaint(Color.RED);
                g2.fillOval(x-10, y-10, 20, 20);
                repaint(x-10, y-10, 20, 20);
            }
        });
    }

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

        if (image == null){
            image = createImage(getWidth(), getHeight());
            g2 = (Graphics2D) image.getGraphics();
            g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            g2.setBackground(COLOR_TRANSPARENT);
            clear();
        }
        Graphics2D g2 = (Graphics2D) g;
        g2.drawImage(image, 0,0, null);
    }

    public void clear(){
        System.out.println("clearing canvas ");
        g2.setComposite(AlphaComposite.Clear);
        g2.setBackground(COLOR_TRANSPARENT);
        g2.setColor(COLOR_TRANSPARENT);
        g2.fillRect(0, 0, getWidth(), getHeight());
        g2.clearRect(0, 0, getWidth(), getHeight());
        g2.setPaint(Color.RED);
        g2.setComposite(AlphaComposite.SrcOver);
        repaint();
    }
}
}

Here is screenshot what I wanted.

Example code updated. Now code should work without any other additional code.

Answer 1

For windows I made a couple of changes:

image = createImage(getWidth(), getHeight());
image = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);

I used a BufferedImage to you can set the alpha values of the image to be transparent.

//public static final Color COLOR_TRANSPARENT = new Color(0,0,0,0);
public static final Color COLOR_TRANSPARENT = new Color(0,0,0,1);

I made the alpha value non-zero, because a value of zero means the Java application won't receive the MouseEvent because it is passed to the application under the window.