Learning Java Program

import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import javax.swing.*;
public class ToolBarDemo extends JPanel 

{  
    public static void main(String[] args) 

    {
        JFrame window = new JFrame("ToolBarDemo");
        ToolBarDemo content = new ToolBarDemo();
        window.setContentPane(content);
        window.pack(); 
        window.setResizable(false);
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        window.setLocation( (screenSize.width - window.getWidth())/2,
                (screenSize.height - window.getHeight())/2 );
        window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        window.setVisible(true);
    }
    private class Display extends JPanel implements MouseListener, MouseMotionListener 

    {
        private BufferedImage OSC;  // Off-screen canvas.
        private Color currentColor = Color.RED;  // Current drawing color.
        private int prevX, prevY;  // Previous mouse position, during mouse drags.
        private BasicStroke stroke;  // Stroke used for drawing.
        Display() { // constructor.
            addMouseListener(this);
            addMouseMotionListener(this);
            setPreferredSize(new Dimension(300,300));
            stroke = new BasicStroke(3,BasicStroke.CAP_ROUND,BasicStroke.JOIN_ROUND);
        }
        void setCurrentColor(Color c) {  // change current drawing color
            currentColor = c;
        }

        void clear() { // clear the drawing area by filling it with white
            if (OSC != null) {
                Graphics g = OSC.getGraphics();
                g.setColor(Color.WHITE);
                g.fillRect(0,0,getWidth(),getHeight());
                g.dispose();
                repaint();
            }
        }
        public void paintComponent(Graphics g) { // just copies OSC to screen
            checkImage();
            g.drawImage(OSC,0,0,null);
        }
        void checkImage() {  // create or resize OSC if necessary
            if (OSC == null) {
                // Create the OSC, with a size to match the size of the panel.
                OSC = new BufferedImage(getWidth(),getHeight(),BufferedImage.TYPE_INT_RGB);
                clear();
            }
            else if (OSC.getWidth() != getWidth() || OSC.getHeight() != getHeight()) {
                BufferedImage newOSC;
                newOSC = new BufferedImage(getWidth(),getHeight(),BufferedImage.TYPE_INT_RGB);
                Graphics g = newOSC.getGraphics();
                g.drawImage(OSC,0,0,getWidth(),getHeight(),null);
                g.dispose();
                OSC = newOSC;
            }
        }
        public void mousePressed(MouseEvent e) {
            prevX = e.getX();
            prevY = e.getY();
        }
        public void mouseDragged(MouseEvent e) {
            Graphics2D g2 = (Graphics2D)OSC.getGraphics();
            g2.setColor(currentColor);
            g2.setStroke(stroke);
            g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            g2.drawLine(prevX,prevY,e.getX(),e.getY());
            g2.dispose();
            repaint();
            prevX = e.getX();
            prevY = e.getY();
        }
        public void mouseReleased(MouseEvent e) { }
        public void mouseMoved(MouseEvent e) { }
        public void mouseClicked(MouseEvent e) { }
        public void mouseEntered(MouseEvent e) { }
        public void mouseExited(MouseEvent e) { }
    }


    private Display display; 

    public ToolBarDemo() {
        setLayout(new BorderLayout(2,2));
        setBackground(Color.GRAY);
        setBorder(BorderFactory.createLineBorder(Color.GRAY,2));
        display = new Display();
        add(display, BorderLayout.CENTER);
        JToolBar toolbar = new JToolBar();
        add(toolbar, BorderLayout.NORTH);
        ButtonGroup group = new ButtonGroup();
        toolbar.add( makeColorRadioButton(Color.RED,group,true) );
        toolbar.add( makeColorRadioButton(Color.GREEN,group,false) );
        toolbar.add( makeColorRadioButton(Color.BLUE,group,false) );
        toolbar.addSeparator(new Dimension(20,20));
        toolbar.add( makeClearButton() );
    }
   
    private JRadioButton makeColorRadioButton(final Color c, ButtonGroup grp, boolean selected) {       
        BufferedImage image = new BufferedImage(30,30,BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();
        g.setColor(Color.LIGHT_GRAY);
        g.fillRect(0,0,30,30);
        g.setColor(c);
        g.fill3DRect(1, 1, 24, 24, true);
        g.dispose();
        Icon unselectedIcon = new ImageIcon(image);
        /* Create an ImageIcon for the selected state of the button. */
        image = new BufferedImage(30,30,BufferedImage.TYPE_INT_RGB);
        g = image.getGraphics();
        g.setColor(Color.DARK_GRAY);
        g.fillRect(0,0,30,30);
        g.setColor(c);
        g.fill3DRect(3, 3, 24, 24, false);
        g.dispose();
        Icon selectedIcon = new ImageIcon(image);
       /* Create and configure the button. */
        JRadioButton button = new JRadioButton(unselectedIcon);
        button.setSelectedIcon(selectedIcon);
        button.addActionListener( new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                // The action for this button sets the current drawing color
                // in the display to c.
                display.setCurrentColor(c);
            }
        });
        grp.add(button);
        if (selected)
            button.setSelected(true);
        return button;
    } // end makeColorRadioButton   
    private JButton makeClearButton() {
        BufferedImage image = new BufferedImage(24,24,BufferedImage.TYPE_INT_RGB);
        Graphics2D g2 = (Graphics2D)image.getGraphics();
        g2.setColor(Color.LIGHT_GRAY);
        g2.fillRect(0,0,24,24);
        g2.setStroke( new BasicStroke(3));
        g2.setColor(Color.BLACK);
        g2.drawLine(4,4,20,20);
        g2.drawLine(4,20,20,4);
        g2.dispose();
        Icon clearIcon = new ImageIcon(image);
        Action clearAction = new AbstractAction(null,clearIcon) {
            public void actionPerformed(ActionEvent evt) {
                display.clear();
            }
        };
        clearAction.putValue(Action.SHORT_DESCRIPTION, "Clear the Display");
        JButton button = new JButton(clearAction);
        return button;
    }
}