Core Swing Components - The Definitive Guide to Java Swing - page 66

Component Pieces
The JComponent class defines many aspects of AWT components that go above and beyond the
capabilities of the original AWT component set. This includes customized painting behavior
and several different ways to customize display settings, such as colors, fonts, and any other
client-side settings.
Painting JComponent Objects
Because the Swing JComponent class extends from the Container class, the basic AWT painting
model is followed: All painting is done through the paint() method, and the repaint() method
is used to trigger updates. However, many tasks are done differently. The JComponent class opti-
mizes many aspects of painting for improved performance and extensibility. In addition, the
RepaintManager class is available to customize painting behavior even further.
■ Note The public void update(Graphics g) method, inherited from Component , is never invoked
on Swing components.
To improve painting performance and extensibility, the JComponent splits the painting
operation into three tasks. The public void paint(Graphics g) method is subdivided into three
separate protected method calls. In the order called, they are paintComponent(g) , paintBorder(g) ,
and paintChildren(g) , with the Graphics argument passed through from the original paint()
call. The component itself is first painted through paintComponent(g) . If you want to customize
the painting of a Swing component, you override paintComponent() instead of paint() . Unless
you want to completely replace all the painting, you would call super.paintComponent() first,
as shown here, to get the default paintComponent() behavior.
public class MyComponent extends JPanel {
protected void paintComponent(Graphics g) {
super.paintComponent(g);
// Customize after calling super.paintComponent(g)
}
...
}
■ Note When running a program that uses Swing components in Java 5.0, the Graphics argument passed
to the paint() method and on to paintComponent() is technically a Graphics2D argument. Therefore,
after casting the Graphics argument to a Graphics2D object, you could use the Java 2D capabilities of the
platform, as you would when defining a drawing Stroke , Shape , or AffineTransform .