关于 JAVA paint(Graphics g) 实在是搞不懂，快要疯了！

大家看一下，以下两段代码有什么不同：
============= 代码块 1==================
public void paint(Graphics g) {
super.paint(g);
paintTest();
//g.setColor(Color.BLACK);
//g.drawOval(409,231,15,15);
//g.fillOval(327,372,15,15);
}
public void paintTest() {
Graphics2D g = (Graphics2D)this.getGraphics();
g.setColor(Color.BLACK);
g.drawOval(409, 231, 15, 15);
g.drawOval(327, 372, 15, 15);
}

============= 代码块 2==================
public void paint(Graphics g) {
super.paint(g);
//paintTest();
g.setColor(Color.BLACK);
g.drawOval(409,231,15,15);
g.fillOval(327,372,15,15);
}

真是搞不懂，中间只是调用了一个函数，结果就会一个能画得出来，另一个却画不出来了。

下面是全部的源代码。共有两个 java 文件
==================TestPanel.java======================

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

public class TestPanel extends JPanel {
BorderLayout borderLayout1 = new BorderLayout();

public TestPanel() {
try {
jbInit();
}
catch(Exception ex) {
ex.printStackTrace();
}
}

public void paint(Graphics g) {
super.paint(g);
paintTest();
//g.setColor(Color.BLACK);
//g.drawOval(409,231,15,15);
//g.fillOval(327,372,15,15);
}

public void paintTest() {
Graphics2D g = (Graphics2D)this.getGraphics();
g.setColor(Color.BLACK);
g.drawOval(409, 231, 15, 15);
g.drawOval(327, 372, 15, 15);
}

void jbInit() throws Exception {
this.setLayout(borderLayout1);
}
}

==================Frame1.java======================

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

public class Frame1 extends JFrame {
BorderLayout borderLayout1 = new BorderLayout();
JPanel jPanel1 = new TestPanel();
JPanel jPanel2 = new JPanel();
JPanel jPanel3 = new JPanel();
JButton jButton2 = new JButton();

public Frame1() {
try {
jbInit();
}
catch(Exception ex) {
ex.printStackTrace();
}
}

void jbInit() throws Exception {
this.getContentPane().setLayout(borderLayout1);
jButton2.setText("Test");
this.getContentPane().add(jPanel1, BorderLayout.CENTER);
this.getContentPane().add(jPanel2, BorderLayout.SOUTH);
jPanel2.add(jButton2, null);
this.getContentPane().add(jPanel3, BorderLayout.NORTH);
}

public static void main(String[] args) {
Frame1 frame = new Frame1();
frame.setLocation(100, 100);
frame.setSize(500, 500);
frame.setVisible(true);
}
}