HACK

import java.awt.Component;
import java.awt.Container;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;

public class BoxLayoutDemo {
public static void addComponentsToPane(Container pane) {
pane.setLayout(new BoxLayout(pane, BoxLayout.Y_AXIS));

addAButton("Button 1", pane);
addAButton("Button 2", pane);
addAButton("Button 3", pane);
addAButton("Long-Named Button 4", pane);
addAButton("5", pane);
}

private static void addAButton(String text, Container container) {
JButton button = new JButton(text);
button.setAlignmentX(Component.CENTER_ALIGNMENT);
container.add(button);
}

private static void createAndShowGUI() {

JFrame frame = new JFrame("BoxLayoutDemo");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

addComponentsToPane(frame.getContentPane());

frame.pack();
frame.setVisible(true);
}

public static void main(String[] args) {

javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
}
}
OUTPUT