main thread

public class ThreadKotak extends Thread {
    // execute application

    private static Kotak shapes2JPanel;

    public static void main(String args[]) {
        // create frame for Shapes2JPanel
        JFrame frame = new JFrame("Drawing 2D Shapes");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        shapes2JPanel = new Kotak();
        frame.add(shapes2JPanel); // add shapes2JPanel to frame
        frame.setBackground(Color.WHITE); // set frame background color
        frame.setSize(400, 400); // set frame size
        frame.setVisible(true); // display frame
        ThreadKotak sh = new ThreadKotak();
        sh.start();
    } // end main

    public void run() {
        while (true) {
            try {
                this.sleep(500);
            } catch (InterruptedException ie) {
                break;
            }
            shapes2JPanel.repaint();
        }
    }
}