Getting the image of a graphics object?

Question

Hi,
I have a graphics object that I want to update on a thread, but i don't want to redraw everything I had on each time I update (I only want to draw new items). in my paintComponent function, I have
public void paintComponent(Graphics g)
          Graphics2D g2 = (Graphics2D)g;
          if (bufferedImage == null)
               bufferedImage = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_RGB);
          g2.drawImage(bufferedImage, 0, 0, null);
          drawBackground(g2);
          addItem(g2);
             g2_ = g2;
     }where drawBackground makes the constant background and addItem draws new points. The problem that I'm having is that it is only drawing new items on the graph and not adding on to the existing image. I think this is because I never set bufferedImage to the current image, but I'm not sure how to do this?
Thanks

Answer 1

demo:
import java.awt.*;
import java.awt.image.*;
import javax.swing.*;
@SuppressWarnings("serial") public class Example extends JComponent {
    private BufferedImage buffer;
        setOpaque(true);
    @Override protected void paintComponent(Graphics g) {
        int w = getWidth(), h = getHeight();
        if (buffer == null ||
            w > buffer.getWidth() || h > buffer.getHeight()) {
            buffer = getGraphicsConfiguration().createCompatibleImage(w, h);
            composeBuffer();
        g.drawImage(buffer, 0, 0, null);
    protected void composeBuffer() {
        Graphics2D g2 = buffer.createGraphics();
        g2.setColor(Color.RED);
        g2.drawLine(0, 0, buffer.getWidth(), buffer.getHeight());
        g2.dispose();
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable(){
            public void run() {
                JComponent comp = new Example();
                comp.setPreferredSize(new Dimension(400,300));
                JFrame f = new JFrame();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.add(comp);
                f.pack();
                f.setLocationRelativeTo(null);
                f.setVisible(true);
}