Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 862

14.5 Two-Dimensional Graphics
In Supplement 3G, we introduced a graphical class called DrawingPanel . The
DrawingPanel was simple enough that you did not need to learn a lot of details
about graphical user interfaces. Now that we're starting to uncover those details, we
can examine how to draw our own two-dimensional graphics manually. This will
help you eventually understand the code for DrawingPanel and even reimplement it
yourself.
Drawing onto Panels
Earlier in this chapter we discussed the JPanel component, which we used as an
invisible container for laying out other components. Panels have one other important
function: They serve as surfaces onto which we can draw. The JPanel object has a
method called paintComponent that draws the panel on the screen. By default this
method draws nothing, so the panel is transparent.
We can use inheritance to change the drawing behavior for a panel. If we want to
make a panel onto which we can draw shapes, we can extend JPanel and override
the paintComponent method. Here is the header of the paintComponent method
we must override:
public void paintComponent(Graphics g)
Its parameter, Graphics g , should look familiar to you if you read Supplement 3G.
Graphics is a class in Java's java.awt package that has methods for drawing shapes,
lines, and images onto a surface. Think of the Graphics object as a pen and the panel
as a sheet of paper.
There is one quirk to keep in mind when you are writing a paintComponent
method. Remember that when you override a method, you replace the superclass
method's previous functionality. We don't want to lose the behavior of paintComponent
from JPanel , though, because it does important things for the panel; we just want to
add additional behavior to it. Therefore, the first thing you should do when you over-
ride paintComponent is to call super.paintComponent and pass it your Graphics
object g as its parameter. (If you don't, you can get strange ghosty afterimages when
you drag or resize your window.)
Here's a small class that represents a panel with two rectangles drawn on it:
1 // A panel that draws two rectangles on its surface.
2
3 import java.awt.*;
4 import javax.swing.*;
5
6 public class RectPanel extends JPanel {
7 public void paintComponent(Graphics g) {
8 super .paintComponent(g); // call JPanel's version