/* * This code is based on an example provided by John Vella, * a tutorial reader. */import javax.swing.*;import javax.swing.event.MouseInputAdapter;import java.awt.*;import java.awt.event.*;import java.util.*;public class ScrollDemo2 extends JPanel {    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;    JPanel drawingArea;    public ScrollDemo2() {        setOpaque(true);        size = new Dimension(0,0);        objects = new Vector();        //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);        //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());        //Put the drawing area in a scroll pane.        JScrollPane scroller = new JScrollPane(drawingArea);        scroller.setPreferredSize(new Dimension(200,200));        //Layout this demo.        setLayout(new BorderLayout());        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();        }    }    public static void main (String args[]) {        JFrame frame = new JFrame("ScrollDemo2");        frame.addWindowListener(new WindowAdapter() {            public void windowClosing(WindowEvent e) {System.exit(0);}        });        frame.setContentPane(new ScrollDemo2());        frame.pack();        frame.setVisible(true);    }}