jbutton按钮上的文本显示在椭圆中,该怎么处理

import java.awt.Graphics;import java.awt.Graphics2D;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.BorderLayout;import java.awt.Color;import java.awt.Font;import javax.swing.JButton;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JOptionPane;import javax.swing.JPanel;public class DrawOral {    public static void main(String[] args)    {        new frame();        }}class frame extends JFrame{    /**     *      */    private static final long serialVersionUID = 1L;    public frame()    {        setSize(400,400);        setLocation(400,200);        button bt=new button("测试");        bt.addActionListener(new ActionListener(){            @Override            public void actionPerformed(ActionEvent e) {                // TODO Auto-generated method stub                JOptionPane.showMessageDialog(frame.this,"hah");            }});        //add(bt);        JPanel panel=new JPanel();                panel.add(bt);        add(panel,BorderLayout.NORTH);        add(new JLabel("点击圆试试"),BorderLayout.CENTER);        this.setVisible(true);    }}class button extends JButton{    /**     *      */    private static final long serialVersionUID = 1L;    public button(String str)    {                 super(str);        }        public void paint(Graphics g)    {        g.setColor(Color.RED);        g.drawOval(0,0,getHeight(),getHeight());        Graphics2D g2=(Graphics2D)g;        g2.setFont(new Font("Arial",Font.ITALIC,getHeight()/2));        g2.setPaint(Color.RED);        g2.drawString("OK ",0,getHeight()/2);            }}