Animate.java

import javax.swing.*;
import java.awt.*;

public class Animate
{
    int x = 1;
    int y = 1;

    public static void main(String[] args)
    {
        Animate gui = new Animate();
        gui.go();
    }

    public void go()
    {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        MyDrawP drawP = new MyDrawP();

        frame.getContentPane().add(drawP);
        frame.setSize(500,270);
        frame.setVisible(true);
        
        for (int i = 0; i < 124; i++,y++,x+=2)
        {
            //x++;
            drawP.repaint();
            try
            {
                Thread.sleep(50);
            }
            catch(Exception ex)
            {
            }
        }
    }

    class MyDrawP extends JPanel
    {   
        public void paintComponent(Graphics g)
        {
            g.setColor(Color.white);
            g.fillRect(0,0,500,250);
            g.setColor(Color.blue);
            g.fillRect(x,y,500-x*2,250-y*2);
        }
    }
}