主题：课件17下载——布局管理器

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

public class exec
{
        public static void main(String [] args)
        {
                JFrame f=new JFrame("Hello!");
                f.setLocation(10,100);
                f.setSize(600,400);
                f.setMinimumSize(new Dimension(200,100));
                
                f.getContentPane().setLayout(new GridLayout(3,3,10,10));
                JButton[] jb=new JButton[9];
                
                for(int i=0;i<jb.length;i++)
                {
                        jb=new JButton(String.valueOf(i));
                        f.getContentPane().add(jb);
                }      

                f.setVisible(true);            
        }      
}
红色显示的部分运行时出错了，可我不知道该怎么修改，帮忙看一下啊