Learning Java Program

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class InfiniteMotion extends JPanel implements ActionListener 

{
    public void drawFrame(Graphics g, int frameNumber, int width, int height) 

   {
        int inset;
        int rectWidth, rectHeight; 
        g.setColor(Color.LIGHT_GRAY);
        g.fillRect(0,0,width,height); 
        g.setColor(Color.BLACK); 
        inset = frameNumber % 15;
        rectWidth = width - 2*inset;
        rectHeight = height - 2*inset;
        while (rectWidth >= 0 && rectHeight >= 0) 

        {
            g.drawRect(inset, inset, rectWidth, rectHeight);
            inset += 15;     
            rectWidth -= 30;
            rectHeight -= 30;
        }
    }
    public static void main(String[] args) 

   {
        JFrame window = new JFrame("Infinite motion");
        InfiniteMotion drawingArea = new InfiniteMotion();
        drawingArea.setBackground(Color.WHITE);
        window.setContentPane(drawingArea);
        drawingArea.setPreferredSize(new Dimension(600,450));
        window.pack();
        window.setLocation(100,50);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setResizable(true);
        Timer frameTimer = new Timer(20,drawingArea);
        window.setVisible(true);
        frameTimer.start();
    }
    private int frameNum;
    public void actionPerformed(ActionEvent evt) 

    {
        frameNum++;
        repaint();
    }
    protected void paintComponent(Graphics g) 

    {  

        super.paintComponent(g); 

        drawFrame(g, frameNum, getWidth(), getHeight()); 

    } 

}