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

public class Swing5 extends JFrame {
    private JButton[] buttons;

    public Swing5() {
        setTitle("Swing5");
        setSize(400,400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        Container pane = getContentPane();
        pane.setLayout(new BoxLayout(pane, BoxLayout.X_AXIS));

        buttons = new JButton[12];
        for (int i = 0; i < buttons.length; ++i) {
            buttons[i] = new JButton(Integer.toString(i));
        }
        
        JPanel topLevel = new JPanel();
        JPanel level2Left = new JPanel();
        JPanel level2Right = new JPanel();
        
        pane.add(topLevel);

        level2Left.setLayout(new BoxLayout(level2Left, BoxLayout.Y_AXIS));
        level2Right.setLayout(new BoxLayout(level2Right, BoxLayout.Y_AXIS));

        topLevel.add(level2Left);
        topLevel.add(level2Right);

        int halfway = buttons.length / 2;
        for (int i = 0; i < halfway; ++i) {
            level2Left.add(buttons[i]);
            level2Right.add(buttons[halfway + i]);
        }
    }

    public static void main(String[] args) {
        Swing5 s = new Swing5();
        s.setVisible(true);
    }
}