Tips using Swing
Source: Various tutorials from Sun

When a component needs to be displayed or redrawn its paint() or update() method is automatically invoked with an appropriate Graphics context - standard components paints (or repaints) automatically, you don't have to change or do anything!!

If you need to paint from an object (i.e. using drawRect() etc.) decide which superclass to use - normally JPanel or more specialized component will do.

Keep all your painting inside paintComponent(Graphics g)
Before performing any custom painting you should paint the components background ("erase everything") by invoking super.paintComponent(Graphics g).
This is extra important if any component is lying behind this one (they may have to be updated as well).

The object of type Graphics provides methods for the painting (drawRect(), drawImage(), drawString() etc.)

repaint() requests that components is to be scheduled to paint itself. Don't call paintComponent(), call repaint() - let the system decide!
When pace is tight the system might combine multiple repaint() requests into a single - you have no guarantee that all your repaint() requests will be performed.

repaint() has 2 useful forms:

The painting area can be reduced using repaint(int, int, int, int) or by using Graphics.setClip().

When using setClip() always remember to restore the original area:

NEVER override paint() when you are using Swing!!
You'll confuse the automatic painting system!
So remember: NEVER override paint() when you are using Swing!!

Prefer Icon to Image when you are using Swing (if possible).

Some components (like JLabel) needs to invoke their setOpaque(true) if background color is to be changed.

To improve performance of image-animation:

• use MediaTracker to load image (e.g. via Icon which uses MediaTracker automatically)
• when loading images across the internet (e.g. in an Applet) store all files (incl. images) in a .jar file