4003-707 Advanced Java Programming

all, section 15.

15. Painting

When a Swing GUI needs to paint itself -- whether for the first time, in response to becoming unhidden, or because it needs to reflect a change in the program's state -- it starts with the highest component that needs to be repainted and works its way down the containment hierarchy. This process is orchestrated by the AWT painting system, and made more efficient and smooth by the Swing repaint manager and double-buffering code.

Swing components generally repaint themselves whenever necessary. When you invoke the setText method on a component, for example, the component should automatically repaint itself and, if appropriate, resize itself. If it doesn't, it's a bug. The workaround is to invoke the repaint method on the component to request that the component be scheduled for painting. If the component's size or position needs to change but doesn't do so automatically, you should invoke revalidate upon the component before invoking repaint.

Like event-handling code, painting code executes on the event-dispatching thread. While an event is being handled, no painting will occur. Similarly, if a painting operation takes a long time, no events will be handled during that time.

Programs should paint only when the painting system tells them to because each occurrence of a component painting itself must execute without interruption. Otherwise, unpredictable results could occur, such as a button being painted as half pressed and half unpressed.

For smoothness, Swing painting is double-buffered by default -- performed to an offscreen buffer and then flushed to the screen once finished. It might slightly help performance if you make a Swing component opaque, so that the Swing painting system can know not to paint anything behind the component. To make a Swing component opaque, invoke setOpaque(true) on the component.

Although their available painting area is always rectangular, non-opaque Swing components can appear to be any shape. A button, for instance, might display itself by painting a filled octagon. The component behind the button (its container, most likely) would then be visible, showing through at the corners of the button's bounds. The button would have to include special hit detection code to avoid acting pressed if the user happens to click on its corners.