帮忙看一道程序题
怎样让结果显示在屏幕的中间啊

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class GridBag29 extends JFrame
{
private JTextField tf;
private JCheckBox cb1,cb2;
private JRadioButton rb1,rb2,rb3;
private JLabel lb1,lb2,lb3;
private JPanel p1,p2,p3;

public void init()
{

p1=new JPanel();
p2=new JPanel();
p3=new JPanel();

lb1=new JLabel("送货地址:");
tf=new JTextField(15);
lb2=new JLabel("货品项目:");
cb1=new JCheckBox("PIZZA");
cb2=new JCheckBox("可乐");
lb3=new JLabel("送货时段:");
rb1=new JRadioButton("上午",false);
rb2=new JRadioButton("下午",false);
rb3=new JRadioButton("晚上",false);

Container c=getContentPane();
c.setLayout(new FlowLayout());
p1.add(lb1);
p1.add(tf);
p2.add(lb2);
p2.add(cb1);
p2.add(cb2);
p3.add(lb3);
p3.add(rb1);
p3.add(rb2);
p3.add(rb3);
c.add(p1);
c.add(p2);
c.add(p3);

setSize(400,200);
setVisible(true);

}
}