When a graphic is complex or is used repeatedly, you can reduce the time it takes to display it by first rendering it to an off-screen buffer and then copying the buffer to the screen. This technique, called double buffering, is often used for animations.

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Note:  When you are rendering into a Swing component, Swing automatically double-buffers the display.

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A BufferedImage can easily be used as an off-screen buffer. To create a BufferedImage whose color space, depth, and pixel layout exactly match the window into which you're drawing, call the Component createImage method. If you need control over the off-screen image's type or transparency, you can construct a BufferedImage object directly and use it as an off-screen buffer.

To draw into the buffered image, you call the BufferedImage createGraphics method to get a Graphics2D object; then you call the appropriate rendering methods on the Graphics2D. All of the Java 2D API rendering features can be used when you're rendering to a BufferedImage that's being used as an off-screen buffer.

When you're ready to copy the BufferedImage to the screen, you simply call drawImage on your component's Graphics2D and pass in the BufferedImage.

The following applet allows the user to drag a rectangle around within the applet window. Instead of rendering the rectangle at every mouse location to provide feedback as the user drags it, a BufferedImage is used as an off-screen buffer. As the rectangle is dragged, it is re-rendered into the BufferedImage at each new location and the BufferedImage is blitted to the screen.

This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

The source code for the applet is in BufferedShapeMover.java.

Here is the code used to render into the BufferedImage and display the image on the screen:

public void updateLocation(MouseEvent e){
    rect.setLocation(last_x + e.getX(),
                     last_y + e.getY());
    ...
    repaint();
    ...
    // In the update method...
    if(firstTime) {
        Dimension dim = getSize();
        int w = dim.width;
        int h = dim.height;
        area = new Rectangle(dim);
        bi = (BufferedImage)createImage(w, h);
        big = bi.createGraphics();
        rect.setLocation(w/2-50, h/2-25);
        big.setStroke(new BasicStroke(8.0f));
        firstTime = false;
    }

    // Clears the rectangle that was previously drawn.
    big.setColor(Color.white);
    big.clearRect(0, 0, area.width, area.height);

    // Draws and fills the newly positioned rectangle
    // to the buffer.
    big.setPaint(strokePolka);
    big.draw(rect);
    big.setPaint(fillPolka);
    big.fill(rect);

    // Draws the buffered image to the screen.
    g2.drawImage(bi, 0, 0, this);
}