绘图程序有些疑问 求指导

/** *  */package Paint.App14_1;import java.awt.Color;import java.awt.Font;import java.awt.Graphics;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import javax.swing.JButton;import javax.swing.JFrame;import javax.swing.JPanel;public class App14_1 extends JFrame {    /**     *      */    private static final long serialVersionUID = 1L;    static JButton jButton1 = new JButton("画圆");    static JButton jButton2 = new JButton("画椭圆");    static Mypanel mypanel = new Mypanel();    static App14_1 jframe = new App14_1();    static int circle = 0;    /**     * @param args     */    public static void main(String[] args) {        // TODO Auto-generated method stub        jframe.setVisible(true);        jframe.setTitle("简单的绘图应用程序");        jframe.setBounds(500, 250, 300, 250);        jframe.setLayout(null);        jframe.add(mypanel);        mypanel.setBounds(0, 0, 300, 250);        jframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    }    static class Mypanel extends JPanel implements ActionListener {        /**         *          */        private static final long serialVersionUID = 1L;        /**         *          */        public Mypanel() {            // TODO Auto-generated constructor stub            add(jButton1);            add(jButton2);            jButton1.addActionListener(this);            jButton2.addActionListener(this);            // jButton1.setBounds(50, 180, 80, 25);            // jButton2.setBounds(150, 180, 80, 25);        }        /*         * (non-Javadoc)         *          * @see         * java.awt.event.ActionListener#actionPerformed(java.awt.event.ActionEvent         * )         */        @Override        public void actionPerformed(ActionEvent e) {            // TODO Auto-generated method stub            JButton button = (JButton) e.getSource();            if (button == jButton1)                circle = 1;            else                circle = 2;            Graphics graphics = this.getGraphics();//把这句注释掉怎么也可以正确运行？            repaint();        }        public void paintComponent(Graphics g) {            super.paintComponent(g);            jButton1.setBounds(50, 180, 80, 25);            jButton2.setBounds(150, 180, 80, 25);            g.setFont(new Font("楷体", Font.ITALIC, 20));            g.setColor(Color.red);            g.drawString("画圆或椭圆", 120, 30);            if (circle == 1)                g.drawOval(100, 70, 70, 70);            else if (circle == 2)                g.drawOval(80, 40, 70, 120);        }    }}