一个作图程序paint函数问题

Question

只是在JPanel绘图的,主类继承了jpanel。paint函数如下:
public void paint(Graphics g){
    super.paint(g);
    Graphics2D g2D=(Graphics2D)g;
    g2D.setColor(Color.red);
    g2D.setStroke(new BasicStroke(1.2f));
    g2D.setFont(new Font("宋体",0,15));
    g2D.drawString("My draw program!",0,-10);
    for(int i=0;i<trace.size()-1;i++){
      g2D.drawLine((int)((Point2D.Double)trace.elementAt(i)).getX(),
                   -(int)((Point2D.Double)trace.elementAt(i)).getY(),
                   (int)((Point2D.Double)trace.elementAt(i+1)).getX(),
                   -(int)((Point2D.Double)trace.elementAt(i+1)).getY());
    }
    g2D.setStroke(new BasicStroke(1.0f));
    g2D.setColor(Color.blue);
    g2D.drawLine(0,0,30,-220);
  }