要用到java.awg.Graphics2D 和 java.awt.BasicStroke

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

class Demo extends JFrame
{
    public Demo()
    {
        this.setContentPane(new MyPanel());
        this.setSize(400,330);
        this.setLocationRelativeTo(null);
        this.setVisible(true);
    }
    
    public static void main(String[]args)
    {
        new Demo();
    }
}

class MyPanel extends JPanel
{
    public void paintComponent(Graphics gg)
    {
        super.paintComponent(gg);
        Graphics2D g = (Graphics2D)gg;
        g.setStroke(new BasicStroke(10));
        g.drawOval(100,50,200,200);
        this.repaint();
    }
}

谢谢你的指点！[em2]