Centrer un JPanel dans un JPanel

// la fenêtre (this) fait 800*600

pan = new JPanel();
pan.setLayout(new BorderLayout());
pan.setBackground(Color.white);
this.add(pan);
        
     pantop = new JPanel();
     pantop.setPreferredSize(new Dimension(800, 50));
     pan.add(pantop, BorderLayout.NORTH);
        
     panmid = new JPanel();
     pan.add(panmid, BorderLayout.CENTER);

     panleft = new JPanel();
     panleft.setPreferredSize(new Dimension(50, 500));
     pan.add(panleft, BorderLayout.WEST);

     panright = new JPanel();
     panright.setPreferredSize(new Dimension(50, 500));
     pan.add(panright, BorderLayout.EAST);
        
     panbot = new JPanel();
     panbot.setPreferredSize(new Dimension(800, 50));
     pan.add(panbot, BorderLayout.SOUTH);
        
this.setContentPane(pan);