Problemer med placering af knapper i boxlayout

Vi har 6 knapper som skal centreres under en tabel. Knapperne skal stå i 2 rækker under og over hinanden.

På øverste række skal der være 2 knapper og på nederste 4.

Hvordan får  vi knapperne centreret?
Hvordan bestemmer man afstanden mellem knapperne horisontalt og vertikalt?

Her er vores kode...

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JPanel;


public class GUIButtons extends JPanel {

    private static final long serialVersionUID = 1L;
    private GUICallback callback;
    private JButton book;
    private JButton exchange;
    private JButton takeShift;
    private JButton print;
    private JButton forward;
    private JButton backward;
   
   
    public GUIButtons(GUICallback callback){
        this.callback = callback;
        this.setLayout(new BorderLayout());
        this.add(getButtons(), BorderLayout.SOUTH);
    }


    private JPanel getButtons() {
       
        JPanel mainPanel = new JPanel(new BorderLayout());
        JPanel northPanel = new JPanel();
        JPanel southPanel = new JPanel();
        northPanel.setLayout(new BoxLayout(northPanel, BoxLayout.X_AXIS));
        southPanel.setLayout(new BoxLayout(southPanel, BoxLayout.X_AXIS));
       
       
        backward = new JButton("<<");
        backward.setPreferredSize(new Dimension(20,20));
        backward.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                //callback.lastWeek();
            }
        });
        northPanel.add(backward, BorderLayout.CENTER);
       
        forward = new JButton(">>");
        forward.setPreferredSize(new Dimension(20,20));       
        forward.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                //callback.nextWeek();
            }
        });
        northPanel.add(forward);

       
        book = new JButton("Book");
        book.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                callback.bookShift();
            }
           
        });
        southPanel.add(book);
       
        exchange = new JButton("Byt");
        exchange.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                callback.exchangeShift();
            }
           
        });
        southPanel.add(exchange);
       
       
        takeShift = new JButton("Overtag");
        takeShift.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                callback.takeShift();
            }
           
        });
        southPanel.add(takeShift);
   
        print = new JButton("Print");
        print.setEnabled(false);
       
        southPanel.add(print);
       
        mainPanel.add(northPanel, BorderLayout.NORTH);
        mainPanel.add(southPanel, BorderLayout.SOUTH);
        return mainPanel;
    }
   
}