import java.awt.Color; 
import java.awt.Graphics; 
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.Ellipse2D;

import javax.swing.*; 

public class testpaint extends JFrame{
    public static void main(String[] args) { 
        testpaint t=new testpaint(); 
        t.setLocation(100, 100); 
        t.setSize(500, 400); 
        t.getContentPane().setBackground(Color.green); 
        t.setVisible(true); 
    } 
    public void paint(Graphics g){
        super.paint(g);
        Graphics2D g2 = (Graphics2D)g;
        Ellipse2D e = new Ellipse2D.Double(10,50,70,70);
        //加上下面這句就能變的圓滑了 
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setPaint(Color.RED);
        g2.draw(e);
        //g2.fill(e);
    }
}