Des petits JPanel en mouvement sur un plus grand JPanel

import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Test {
    
    private static void pause(long millis) {
        try {
            Thread.sleep(millis);
        }
        catch (Exception e) {
        }
    }
    
    public static void main(String[] args) {

        final Icon iconHorz = new ImageIcon("C:/rouge.png");
        final JLabel horz = new JLabel(iconHorz);
        horz.setBounds(0, 0, iconHorz.getIconWidth(), iconHorz.getIconHeight());
        
        final Icon iconVert = new ImageIcon("C:/bleu.png");
        final JLabel vert = new JLabel(iconVert);
        vert.setBounds(0, 0, iconVert.getIconWidth(), iconVert.getIconHeight());
        
        final JFrame frame = new JFrame();
        frame.setSize(200,200);
        frame.setLayout(null);
        frame.add(horz);
        frame.add(vert);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        new Thread() {
            @Override
            public void run() {
                while (true) {
                    while (horz.getX() < frame.getContentPane().getWidth()-horz.getWidth()) {
                        horz.setLocation(horz.getX()+1, horz.getY());
                        pause(10);
                    }
                    while (horz.getX() > 0) {
                        horz.setLocation(horz.getX()-1, horz.getY());
                        pause(5);
                    }
                }
            }
        }.start();
        
        new Thread() {
            @Override
            public void run() {
                while (true) {
                    while (vert.getY() < frame.getContentPane().getHeight()-vert.getHeight()) {
                        vert.setLocation(vert.getX(), vert.getY()+1);
                        pause(10);
                    }
                    while (vert.getY() > 0) {
                        vert.setLocation(vert.getX(), vert.getY()-1);
                        pause(5);
                    }
                }
            }
        }.start();
    }
}