public class DemoScrollBar extends JApplet {

    private Dimension size; // indicates size taken up by graphics
    private Vector objects; // rectangular coordinates used to draw graphics

    private final Color colors[] = {    Color.red, Color.blue,   Color.green,
                                     Color.orange, Color.cyan, Color.magenta,
                                   Color.darkGray, Color.yellow };
    private final int color_n = colors.length;

    Container container = null;
    JPanel         drawingArea;

    public void init() {

        // 1. Get a handle on the applet's content pane.

        container = this.getContentPane();

        size    = new Dimension(0,0);
        objects = new Vector();

        // 2.  Set up the instructions.

        JLabel instructionsLeft  = new JLabel( "Click left mouse button to place a circle.");
        JLabel instructionsRight = new JLabel( "Click right mouse button to clear drawing area.");
        JPanel instructionPanel  = new JPanel(new GridLayout(0,1));
        instructionPanel.add(instructionsLeft);
        instructionPanel.add(instructionsRight);

        // 3.  Set up the drawing area.

        drawingArea = new JPanel() {
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);

                Rectangle rect;
                for (int i = 0; i < objects.size(); i++) {
                    rect = (Rectangle)objects.elementAt(i);
                    g.setColor(colors[(i % color_n)]);
                    g.fillOval(rect.x, rect.y, rect.width, rect.height);
                }
            }
        };

        drawingArea.setBackground(Color.white);
        drawingArea.addMouseListener(new MyMouseListener());

        // 4. Put the drawing area in a scroll pane.

        JScrollPane scroller = new JScrollPane(drawingArea);
        scroller.setPreferredSize(new Dimension(200,200));

        // 5. Use border layout (default for an applet) 

        add(instructionPanel, BorderLayout.NORTH);
        add(scroller, BorderLayout.CENTER);

    }

    class MyMouseListener extends MouseInputAdapter {
        final int W = 100;
        final int H = 100;

        public void mouseReleased(MouseEvent e) {
            boolean changed = false;
            if (SwingUtilities.isRightMouseButton(e)) {

                // This will clear the graphic objects.

                objects.removeAllElements();
                size.width=0;
                size.height=0;
                changed = true;
            } else {
                int x = e.getX() - W/2;
                int y = e.getY() - H/2;
                if (x < 0) x = 0;
                if (y < 0) y = 0;
                Rectangle rect = new Rectangle(x, y, W, H);
                objects.addElement(rect);
                drawingArea.scrollRectToVisible(rect);

                int this_width = (x + W + 2);
                if (this_width > size.width)
                    {size.width = this_width; changed=true;}

                int this_height = (y + H + 2);
                if (this_height > size.height)
                    {size.height = this_height; changed=true;}
            }
            if (changed) {

                //Update client's preferred size because 
                //the area taken up by the graphics has
                //gotten larger or smaller (if cleared).

                drawingArea.setPreferredSize(size);

                //Let the scroll pane know to update itself
                //and its scrollbars.
                drawingArea.revalidate();
            }

            drawingArea.repaint();
        }
    }
}