How can get a Graphics to draw line on screen?

Question

How can get a Graphics to draw line on screen?
Now, I can get a Graphics to draw line based on component. For example JPanel, but I want to get a Graphics to draw line on screen.

Answer 1

By drawing on the screen, I assume you mean drawing outside the bounds of a top-level window like
JFrame or JDialog. You can't do that. At least, without going native and even then that's a dodgey thing
for any platform to let you do. One thing you can do is simulate it with a robot's screen capture:
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import javax.swing.*;
public class X {
    public static void main(String[] args) throws Exception {
        Rectangle bounds = GraphicsEnvironment.getLocalGraphicsEnvironment().getMaximumWindowBounds();
        BufferedImage image = new Robot().createScreenCapture(bounds);
        Graphics2D g2 = image.createGraphics();
        g2.setStroke(new BasicStroke(20));
        g2.setPaint(Color.RED);
        g2.drawLine(0, 0, bounds.width, bounds.height);
        g2.drawLine(bounds.width, 0, 0, bounds.height);
        g2.dispose();
        JLabel label = new JLabel(new ImageIcon(image));
        label.addMouseListener(new MouseAdapter(){
            public void mousePressed(MouseEvent evt) {
                System.exit(0);
        JFrame f = new JFrame();
        f.setUndecorated(true);
        f.getContentPane().add(label);
        f.setBounds(bounds);
        f.setVisible(true);
}