Painting in swing. repaint doesn't work.

Hi, today I hve a problem with painting. I created a class Szescian
that extends JPanel and implements Runnable. I want that class to
paint a square that moves. I move it using a thread. The problem is
that when i use repaint in the thread the screen doesn't repaints( the
compiler doesn't jump tu paintComponent(gDC) method) Here is how i
create this object in the class with frame

private Szecian s1;

//COnstructor
public Main(){
.........
        s1 = new Szescian();
        s1.setLocation(0,0);
        s1.setOpaque(true);
        s1.setVisible(true);
        s1.setSize(panelSzescian.getSize());;
        s1.setOpaque(true);
        panelSzescian.add(s1); // a JPanel to which i add s1
         s1.uruchom(); //start the thread
.........
}

and here is my Szescian class:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import java.io.*;
import javax.imageio.*;

public class Szescian extends JPanel implements Runnable{
    public int x,y;
    public Rectangle rect;
    public Szescian(){
        x=0; y=0;
        rect = new Rectangle(0,0,getWidth(), getHeight());
    }

    public void run(){
        for(int i=0; i<50; i++)
        {
            x=i; y=i;

            System.out.println(i);

            try
            {
                Thread.sleep(100);
            }
            catch(InterruptedException e){e.printStackTrace();}
           // repaint(rect);
            repaint();

        }
    }

    protected void paintComponent(Graphics gDC)
    {
        // podwojne buforowanie
         super.paintComponent( gDC );
        Image img = createImage(getWidth(), getHeight());
        Graphics g = img.getGraphics();
        g.setColor(Color.yellow);
        g.drawRect(x,y,20,20);
        g.fillRect(x,y,20,20);
        //gDC.drawImage(img,0,0,getWidth(), getHeight(),this);//
wyswietla obraz z bufora
        gDC.drawImage(img,0,0,this);
    }

    protected void paintBorder(){

    }
    protected void paintChildren(){

    }

    public void uruchom(){
        Runnable r = new Szescian();
        Thread t = new Thread(r);
        t.start();
    }
}