新手提问：java JFrame.add()与JFrame.setcontentPane()区别

package swt;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Demo_13 extends JFrame{
JPanel pane1,pane2,pane3;
JButton button;
JLabel label01,label02;
JTextField jxt01,jxt02;
public static void main(String[] args){
new Demo_13();
}
public Demo_13(){
JPanel pane1=new JPanel();
JPanel pane2=new JPanel();
JPanel pane3=new JPanel();
JButton button=new JButton("登录");
JLabel label01=new JLabel("姓名");
JLabel label02=new JLabel("密码");
JTextField jxt01=new JTextField(10);
JTextField jxt02=new JTextField(10);
pane1.add(label01);
pane1.add(jxt01);
pane2.add(label02);
pane2.add(jxt02);
pane3.add(button);
this.add(pane1);
this.add(pane2);
this.add(pane3);
//this.setContentPane(pane1);
//this.setContentPane(pane2);
//this.setContentPane(pane3);
this.setLayout(new GridLayout(3,1));
this.setTitle("学生管理系统");
this.setSize(300, 300);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setVisible(true);

}
}
-----------------------------------
当我用
this.setContentPane(pane1);
this.setContentPane(pane2);
this.setContentPane(pane3);
替换this.add()方法时候运行结果如图：

用this.add()方法运行结果正常

add()与setContentPane()区别在哪里？