Learning Java Program

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

{
    public static void main(String[] args) 

    {
        JFrame window = new JFrame("TransparencyDemo");
        TransparencyDemo content = new TransparencyDemo();
        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 JSlider triangleTransparency;
    private JSlider ovalTransparency;
    private JSlider rectangleTransparency;
    private JSlider textTransparency;

    private Font textFont = new Font("Serif",Font.BOLD,42);

    private JPanel display = new JPanel() {
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            Color triangleColor = new Color(255,0,0,triangleTransparency.getValue());
            Color ovalColor = new Color(0,255,0,ovalTransparency.getValue());
            Color rectangleColor = new Color(0,0,255,rectangleTransparency.getValue());
            Color textColor = new Color(0,0,0,textTransparency.getValue());
            g.setColor(triangleColor);
            g.fillPolygon(new int[] { scaleX(0.25), scaleX(0.7), scaleX(0.1) },
                    new int[] { scaleY(0.1), scaleY(0.7), scaleY(0.7) },
                    3);
            g.setColor(ovalColor);
            g.fillOval(scaleX(0.3),scaleY(0.45),getWidth()/2,getHeight()/2);
            g.setColor(rectangleColor);
            g.fillRect(scaleX(0.4),scaleY(0.15),getWidth()/2,getHeight()/2);
            FontMetrics metrics = g.getFontMetrics(textFont);
            int lineWidth1 = metrics.stringWidth("Transparency");
            int lineWidth2 = metrics.stringWidth("Demo");
            int textHeight = metrics.getHeight() + metrics.getAscent();
            int topSkip = (getHeight() - textHeight) / 2;
            int leftSkip1 = (getWidth() - lineWidth1) / 2;
            int leftSkip2 = (getWidth() - lineWidth2) / 2;
            g.setColor(textColor);
            g.setFont(textFont);
            g.drawString("Transparency", leftSkip1, topSkip + metrics.getAscent());
            g.drawString("Demo", leftSkip2, topSkip + metrics.getAscent() + metrics.getHeight());
        }
        private int scaleX(double x) {
            return (int)(x * getWidth());
        }
        private int scaleY(double y) {
            return (int)(y * getHeight());
        }
    };
   
    private ChangeListener listener = new ChangeListener() {
        public void stateChanged(ChangeEvent arg0) {
            display.repaint();
        }
    };

    public TransparencyDemo() {
        setLayout(new BorderLayout(3,3));
        setBorder(BorderFactory.createLineBorder(Color.GRAY,2));
        setBackground(Color.GRAY);
        display.setBackground(Color.WHITE);
        display.setPreferredSize(new Dimension(400,300));
        add(display,BorderLayout.CENTER);
        JPanel bottom = new JPanel();
        bottom.setLayout(new GridLayout(4,2,3,3));
        add(bottom,BorderLayout.SOUTH);
        bottom.add( new JLabel("  Triangle Transparency:") );
        triangleTransparency = new JSlider(0,255);
        triangleTransparency.addChangeListener(listener);
        bottom.add( triangleTransparency );
        bottom.add( new JLabel("  Oval Transparency:") );
        ovalTransparency = new JSlider(0,255);
        ovalTransparency.addChangeListener(listener);
        bottom.add( ovalTransparency );
        bottom.add( new JLabel("  Rectangle Transparency:") );
        rectangleTransparency = new JSlider(0,255);
        rectangleTransparency.addChangeListener(listener);
        bottom.add( rectangleTransparency );
        bottom.add( new JLabel("  Text Transparency:") );
        textTransparency = new JSlider(0,255);
        textTransparency.addChangeListener(listener);
        bottom.add( textTransparency );
    }
}