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Tip of the Day
Language: Java
Expertise: Advanced
Mar 12, 2001

The Paint Protocol


The AWT Component.paint and Component.update methods take a Graphics object as a parameter. This object should not be retained, because it might be a transient object valid only during the paint. The AWT implementation is free to destroy that Graphics object after the paint method returns, which makes it pointless (and dangerous) to retain the object.

Do not retain the paint Graphics object. In general, it is not safe to paint outside a call to update() (or paint(), which is called by the default Component.update()). If you do need a long-lived Graphics object, you should create a new one from the argument value. Be warned, however, that this might fail on some platforms:
 
Graphics myGraphics = null; // retained
void paint(Graphics graphics)
{
 if (myGraphics == null)
 {
   myGraphics = graphics.create();
 }
// painting code goes here
}
Chakravarthy Inampudi
 
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