java基础问题请教！

　　import java.awt.*;import java.awt.event.*;import javax.swing.*;public class GridLayoutDemo extends JFrame implements ActionListener{ private JButton buttons[]; private final String names[]={"one","two","three","four","five","six"}; private boolean flag = true; private Container container; private GridLayout grid1,grid2; public GridLayoutDemo() { super("GridLayout Demo"); grid1=new GridLayout(2,3,5,5); grid2=new GridLayout(3,2); Container container=getContentPane(); container.setLayout(grid1); buttons=new JButton[names.length]; for(int count=0;count<names.length;count  ) { buttons[count]=new JButton(names[count]); buttons[count].addActionListener(this); container.add(buttons[count]); } this.setSize(300,150); this.setVisible(true); } public void actionPerformed(ActionEvent e) { if(flag) container.setLayout(grid2); else container.setLayout(grid1); flag = !flag; container.validate(); } public static void main(String agrs[]) { GridLayoutDemo application=new GridLayoutDemo(); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); }}