Learning Java Program

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class StrokeDemo extends JPanel 

{
    public static void main(String[] args) 

    {
        JFrame window = new JFrame("Click and Drag; Right-click for Rectangles");
        StrokeDemo content = new StrokeDemo();
        window.setContentPane(content);
        window.pack(); 
        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 

    {
        Stroke stroke;  // The stroke used for drawing in this panel.
        boolean antialiased;   // Should antialiasing be used?
        boolean drawLine = true;   // Should a line be drawn, or a rectangle?
        int x1, y1, x2, y2;  // Endpoints of line or corners of rectangle.
        Display(Stroke s, boolean a) 

        {
            stroke = s;
            antialiased = a;
            x1 = y1 = 15;
            x2 = 80;
            y2 = 85;
            setPreferredSize(new Dimension(100,100));
            setBackground(Color.WHITE);
        }
        public void paintComponent(Graphics g) 

        {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D)g;
            g2.setStroke(stroke);
            if (antialiased) 

            {
                g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                        RenderingHints.VALUE_ANTIALIAS_ON);
            }
            if (drawLine)
                g2.drawLine(x1,y1,x2,y2);
            else 

            {
                int a = Math.min(x1,x2);
                int b = Math.min(y1,y2);
                int w = Math.abs(x1 - x2);
                int h = Math.abs(y1 - y2);
                g2.drawRect(a,b,w,h);
            }
        }
    }
    private class MouseHandler implements MouseListener, MouseMotionListener

    {
        public void mousePressed(MouseEvent e) 

        {
            for (int i = 0; i < 5; i++)
                for (int j = 0; j < 3; j++) 

                {
                    displays[i][j].x1 = e.getX();
                    displays[i][j].x2 = e.getX();
                    displays[i][j].y1 = e.getY();
                    displays[i][j].y2 = e.getY();
                    displays[i][j].drawLine = ! e.isMetaDown();
                    displays[i][j].repaint();
                }
        }
        public void mouseDragged(MouseEvent e) 

        {
            for (int i = 0; i < 5; i++)
                for (int j = 0; j < 3; j++) 

                {
                    displays[i][j].x2 = e.getX();
                    displays[i][j].y2 = e.getY();
                    displays[i][j].repaint();
                }
        }
        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[][] displays = new Display[5][3];
    public StrokeDemo() 

    {
        setLayout(new GridLayout(3,5,3,3));
        setBorder(BorderFactory.createLineBorder(Color.GRAY,3));
        setBackground(Color.GRAY);
        displays[0][0] = new Display(new BasicStroke(1), false);
        displays[1][0] = new Display(new BasicStroke(2), false);
        displays[2][0] = new Display(new BasicStroke(5), false);
        displays[3][0] = new Display(new BasicStroke(10), false);
        displays[4][0] = new Display(new BasicStroke(20), false);
        displays[0][1] = new Display(new BasicStroke(1,
                BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND), true);
        displays[1][1] = new Display(new BasicStroke(2,
                BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND), true);
        displays[2][1] = new Display(new BasicStroke(5,
                BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND), true);
        displays[3][1] = new Display(new BasicStroke(10,
                BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND), true);
        displays[4][1] = new Display(new BasicStroke(20,
                BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND), true);
        displays[0][2] = new Display(new BasicStroke(1, BasicStroke.CAP_BUTT,
                BasicStroke.JOIN_BEVEL, 10, new float[] {5,5}, 0), true);
        displays[1][2] = new Display(new BasicStroke(2, BasicStroke.CAP_BUTT,
                BasicStroke.JOIN_BEVEL, 10, new float[] {5,5}, 0), true);
        displays[2][2] = new Display(new BasicStroke(5, BasicStroke.CAP_BUTT,
                BasicStroke.JOIN_BEVEL, 10, new float[] {5,5}, 0), true);
        displays[3][2] = new Display(new BasicStroke(10, BasicStroke.CAP_BUTT,
                BasicStroke.JOIN_BEVEL, 10, new float[] {5,5}, 0), true);
        displays[4][2] = new Display(new BasicStroke(20, BasicStroke.CAP_BUTT,
                BasicStroke.JOIN_BEVEL, 10, new float[] {5,5}, 0), true);
        MouseHandler listener = new MouseHandler();
        for (int row = 0; row < 3; row++)
            for (int col = 0; col < 5; col++) 

            {
                add(displays[col][row]);
                displays[col][row].addMouseListener(listener);
                displays[col][row].addMouseMotionListener(listener);
            }
    }
}