Trying to draw a 2D shape, but Graphics2D is null, what am I doing wrong?

Question

At this point, I just want a shape to appear. But when I call draw(...), I get a NullPointer. Turns out when I call getGraphics() on the input JFrame (which does show up properly when the rest is commented out), I get nothing. Maybe it's the casting, I don't know enough about this issue to really say. If anyone out there has worked with 2D primitives and can shed some light on the problem, I would appreciate it.
Thanks for any help.
public class BatteryDisplay extends JPanel {
     Graphics2D g2d;
     Rectangle2D.Double rect1;
     JFrame jf;
     public BatteryDisplay(JFrame jf)
          this.jf = jf;
                // Is there another way to get the graphics object?
          g2d = (Graphics2D) jf.getGraphics();
          createRectangle();
          drawRectangle();
     public void createRectangle()
          rect1 = new Rectangle2D.Double(100,100,200,50);
          System.out.println(rect1.height);
                // The next line returns the NullPointer, so g2d is NOT initalized
          System.out.println(g2d);
     public void drawRectangle()
          this.g2d.draw(rect1);
}

Answer 1

Your painting functions should be initiated from an overridden paintComponent, not from the constructor. A graphics object will be provided to you there.
[http://java.sun.com/docs/books/tutorial/uiswing/painting/index.html]