I'm writing a simple (at this point) drawing program that allows the
user to draw different shapes such as lines, rectangles etc.

The drawings need to be pretty precise so I want to display a grid on
my JPanel, and allow the user to draw objects over the grid.

I have no problems drawing the grid using the Graphics2D object I get
from my offscreen BufferedImage, i.e.,

public void buildGridComponent(Graphics2d g2) {
          Dimension d = getSize();
          Graphics2D g2 = (Graphics2D)gridBufImg.getGraphics();
          for(double x = 0.0; x < d.width; x++) {
           if(x % 10 == 0) {
               Line2D l = new Line2D.Double(new Point2D.Double(x, 0), new
Point2D.Double(x, d.getHeight()));
               g2.draw(l);
           }
       }
              for(double y = 0.0; y < d.width; y++) {
                  if(y % 10 == 0) {
               Line2D l2 = new Line2D.Double(new Point2D.Double(0, y), new
Point2D.Double(d.getWidth(), y));
               g2.draw(l2);
           }
       }
}

But this is incredibly inefficient when drawing.  Painting slows down
so much that my mouse will ghost from the ends of my lines.  Its slow
and choppy.

So, my idea was to just create another BufferedImage, get it's
Graphics2D object, draw to that object, and just draw the BufferedImage
whenever the grid button is toggled.  For some reason though, I'm
blowing up when trying to draw this image.

gridBufImg = new BufferedImage(d.width, d.height,
BufferedImage.TYPE_BYTE_GRAY );
Graphics2D g2 = (Graphics2D)gridBufImg.getGraphics();

When I try and paint this BufferedImage to my offscreen Image I get a
NullPointerException.

java.lang.NullPointerException
       at sun.java2d.pipe.DrawImage.copyImage(Unknown Source)
       at sun.java2d.pipe.DrawImage.copyImage(Unknown Source)
       at sun.java2d.pipe.ValidatePipe.copyImage(Unknown Source)
       at sun.java2d.SunGraphics2D.drawImage(Unknown Source)
       at sun.java2d.SunGraphics2D.drawImage(Unknown Source)

Any hints?