Como desenhar na tela

import java.awt.Color;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.RenderingHints;
import java.awt.geom.AffineTransform;
import java.util.Calendar;
import javax.swing.JFrame;

/**
 *
 * @author http://www.jroller.com/page/adrianromero?entry=jugando_con_java_2d
 */
public class Main extends JFrame {
    
    public Main() {
        this.setDefaultCloseOperation( this.EXIT_ON_CLOSE );
        setSize(400,400);
    }
    
    public static void main(String[] args) {
        // TODO code application logic here
        new Main().setVisible(true);
    }
    
    public void paint(Graphics g) {
        int width = getWidth();
        int height = getHeight();
        
        java.util.Date data = new java.util.Date();
        java.util.Calendar cal = java.util.Calendar.getInstance();
        cal.setTime(data);
        
        double dhour = (double) cal.get( java.util.Calendar.HOUR_OF_DAY );
        double dminute = (double) cal.get( java.util.Calendar.MINUTE);
        double dsecond = (double) cal.get( java.util.Calendar.SECOND);
        
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        
        Paint oldPainter = g2.getPaint();
        AffineTransform oldt = g2.getTransform();
        
        g2.setPaint(new GradientPaint(0, 0, Color.WHITE, width, 0, Color.LIGHT_GRAY));
        g2.fill(g2.getClip());
        
        int icenterx = width / 2;
        int icentery = height / 2;
        int iradius = Math.min(icenterx, icentery);
        
        g2.transform(AffineTransform.getTranslateInstance(icenterx, icentery));
        g2.transform(AffineTransform.getScaleInstance(iradius / 1100.0 , iradius / 1100.0));
        AffineTransform mytrans = g2.getTransform();
        
        g2.setPaint(new GradientPaint(-1200, -1200, Color.BLUE, 1200, 1200, Color.CYAN));
        g2.fillOval(-1000, -1000, 2000, 2000);
        g2.setPaint(new GradientPaint(-1200, -1200, Color.CYAN, 1200, 1200, Color.BLUE));
        g2.fillOval(-900, -900, 1800, 1800);
        g2.setColor(Color.BLACK);
        g2.drawOval(-1000, -1000, 2000, 2000);
        
        for (int i = 0; i < 60; i++) {
            g2.setColor(Color.WHITE);
            g2.fillRect(900, -5 , 50, 10);
            g2.transform(AffineTransform.getRotateInstance(Math.PI / 30.0));
        }
        
        g2.setTransform(mytrans);
        for (int i = 0; i < 12; i++) {
            g2.setColor(Color.WHITE);
            g2.fillRect(800, -15 , 150, 30);
            // g2.setColor(Color.BLACK);
            // g2.drawRect(800, -15 , 150, 30);
            g2.transform(AffineTransform.getRotateInstance(Math.PI / 6.0));
        }
        
        g2.setTransform(mytrans);
        g2.transform(AffineTransform.getRotateInstance((dhour + dminute / 60.0) * Math.PI / 6.0)); // Poner hora
        g2.setColor(Color.WHITE);
        g2.fillPolygon(new int[]{0, -35, 0, 35}, new int[]{100, 0, -600, 0}, 4);
        g2.setColor(Color.DARK_GRAY);
        g2.drawPolygon(new int[]{0, -35, 0, 35}, new int[]{100, 0, -600, 0}, 4);
        
        g2.setTransform(mytrans);
        g2.transform(AffineTransform.getRotateInstance((dminute) * Math.PI / 30.0)); // Poner minutos
        g2.setColor(Color.WHITE);
        g2.fillPolygon(new int[]{0, -35, 0, 35}, new int[]{100, 0, -900, 0}, 4);
        g2.setColor(Color.DARK_GRAY);
        g2.drawPolygon(new int[]{0, -35, 0, 35}, new int[]{100, 0, -900, 0}, 4);
        
        g2.setTransform(mytrans);
        g2.transform(AffineTransform.getRotateInstance(dsecond * Math.PI / 30.0)); // Poner segundos
        g2.setColor(Color.YELLOW);
        g2.fillPolygon(new int[]{-15, 0, 15}, new int[]{200, -900, 200},  3);
        g2.setColor(Color.DARK_GRAY);
        g2.drawPolygon(new int[]{-15, 0, 15}, new int[]{200, -900, 200},  3);
        
        g2.setTransform(mytrans);
        g2.setColor(Color.YELLOW);
        g2.fillOval(-25, -25, 50, 50);
        g2.setColor(Color.DARK_GRAY);
        g2.drawOval(-25, -25, 50, 50);
        g2.setColor(Color.WHITE);
        g2.fillOval(-10, -10, 20, 20);
        g2.setColor(Color.BLACK);
        g2.drawOval(-10, -10, 20, 20);
        
        g2.setTransform(oldt);
        g2.setPaint(oldPainter);
    }
}