Java GroupLayout nest

import java.awt.Container; 
import javax.swing.JFrame; 
import javax.swing.JButton; 
import javax.swing.GroupLayout; 
import static javax.swing.GroupLayout.Alignment.*; 

public class Main { 
    public static void main(String[] args) { 
        JFrame frame = new JFrame("Nested Groups in GroupLayout"); 
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
        Container contentPane = frame.getContentPane(); 

        // Set the content's pane layout to GroupLayout 
        GroupLayout groupLayout = new GroupLayout(contentPane); 
        groupLayout.setAutoCreateGaps(true); 
        groupLayout.setAutoCreateContainerGaps(true); 
        contentPane.setLayout(groupLayout); 

        // Add four JButtons to the content pane 
        JButton b1 = new JButton("Button 1"); 
        JButton b2 = new JButton("Little Bigger Button 2"); 
        JButton b3 = new JButton("3"); 
        JButton b4 = new JButton("Button 4"); 

        groupLayout.setHorizontalGroup( 
            groupLayout.createSequentialGroup() 
            .addGroup(groupLayout.createParallelGroup(LEADING) 
                .addComponent(b1) /* ww  w .j  a v a2s  . c  o m*/
                .addComponent(b3)) 
            .addGroup(groupLayout.createParallelGroup(TRAILING) 
                .addComponent(b2) 
                .addComponent(b4))  
        ); 

        groupLayout.setVerticalGroup( 
            groupLayout.createSequentialGroup() 
            .addGroup(groupLayout.createParallelGroup(BASELINE) 
                .addComponent(b1) 
                .addComponent(b2)) 
            .addGroup(groupLayout.createParallelGroup(BASELINE) 
                .addComponent(b3) 
                .addComponent(b4)) 
        ); 

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