Hello,

I have read many posts and articles saying "don't ever use
getGraphics() in Swing JComponents... always use paintComponent()!" I
understand the motivation for such statements: the paintComponent()
method should be used to implement painting, in order to ensure
correct behavior in all situations.

However, my question is: is it OK, from within an event handler, to
paint using getGraphics() PURELY AS AN OPTIMIZATION, AS LONG AS
paintComponent() does the right thing?

An example:

class MyComponent extends JComponent
{
 ArrayList thingsToPaint=new ArrayList();
 public void mySwingEventHandler(SomeEvent e)
 {
   //update thingsToPaint so that paintComponent
   //works correctly
   thingsToPaint.add(somethingNewToPaint);
   //try painting directly
   Graphics g = getGraphics();
   if(g != null)
   {
      //we have a graphics, so paint the new
      //thing directly.
      somethingNewToPaint.drawOn(g);
      g.dispose();
   }
   else
   {
      //No graphics object available... so just
      //call repaint().
      repaint();
   }
 }
 public void paintComponent(Graphics g)
 {
   //paint everything
   int i, size = thingsToPaint.size();
   for(i=0; i < size; ++i)
   {
     ((ThingToPaint) thingsToPaint.get(i)).drawOn(g);
   }
 }    
};

I actually have a good reason for doing this. I am writing a simple
image manipulation program, and I need to implement a rectanglar
selection facility. In order to accomplish this, I need to listen to
mouseDragged events and repaint the rectangle on the image at each new
position. Swing forces you to re-paint the entire image each time
(i.e. no incremental updates), which is annoyingly slow for this
particular problem. And I don't want to turn off double buffering.

If I use getGraphics() directly (as in the example above), I can just
update a tiny section of the image, and it is blindingly fast and
smooth. I have tested this, and it works wonderfully in my limited
tests. But I don't want to do it if it will be problematic down the
line.

So, is it OK to use getGraphics() if the full repainting functionality
is available in paintComponent()? Or is there some other reason not to
use getGraphics()?

Thanks,

Mike Carrato