JLabel paints dots

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

public class Demo
{
    JFrame jf;
    JLabel[] labels;
    JPanel panel;

    public Demo()
    {
        jf = new JFrame();
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        labels = new JLabel[10];
        Box vbox = Box.createVerticalBox();
        for (int i = 0; i < 10; i++)
        {
            labels[i] = new JLabel();
            vbox.add(labels[i]);
        }
        panel = new JPanel();
        panel.add(vbox);
        jf.add(panel);
        jf.setSize(300, 250);
        jf.setVisible(true);
    }
    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new DemoRunnable());
    }
    public void updateState()
    {
        for (JLabel l : labels)
        {
            if (Math.random() > 0.5)
                l.setText("777777777777777777777777777777777777");
            else
                l.setText("10000000000000000000000000000000000000");
        }
    }
}
class DemoRunnable implements Runnable
{
    Demo demo;

    DemoRunnable()
    {
        this.demo = new Demo();
    }
    @Override
    public void run()
    {
        Thread t = new Thread(new Runnable()
        {

            @Override
            public void run()
            {
                while (true)
                {
                    try
                    {
                        Thread.sleep(0);
                    }
                    catch (InterruptedException e)
                    {
                        e.printStackTrace();
                    }
                    demo.updateState();
                }
            }
        });
        t.start();

    }
}

I see such effect when this program is perfomed. Is it possible to eliminate it(zeroes must be instead dots)?