Media: Graphics, Audio,Video - Java - page 467

on the screen (see Drawing Text ). There are also drawLine() —which draws straight line
segments— setColor / getColor , setFont / getFont , and many other methods. Too many to
list here, in fact; see the online documentation for java.awt.Graphics .
When to draw?
In the past, a common beginner's mistake was call getGraphics() and call the Graphics
object's drawing methods from within a main program or the constructor of a Component
subclass. Fortunately, we now have any number of books to tell us that the correct way to
draw anything is with your component's paint method. Why? Because you can't draw in a
window until it's actually been created and (on most window systems) mapped to the screen,
which takes much more time than your main program or constructor has. The drawing code
needs to wait patiently until the window system notifies the Java runtime that it's time to
paint the window.
Where do you put your drawing code? This is one situation where you need to think about
AWT versus Swing. AWT, the basic windowing system, uses a method called paint() . This
method is still available in Swing, but due to interaction with borders and the like, it is re-
commended that you override paintComponent() instead. Both are called with a single ar-
gument of type Graphics . Your paintComponent() should start by calling su-
per.paintComponent() with the same argument to ensure that components are painted in
proper back-to-front order, whereas paint() should not call its parent. Some examples in
this chapter use paint() and others use paintComponent() ; the latter also usually extend
JPanel . This allows better interaction with Swing, and also allows you to place these as the
main component in a JFrame by calling setContentPane() , which eliminates an extra layer
of container. ( JFrame 's ContentPane is discussed in Displaying GUI Components . )
Showing Graphical Components Without Writing Main
Problem
You don't want to have to write a little main program with a frame each time you write a
subclass of Component .