Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 589

Graphics2D
Use of inheritance is prevalent in the Java class libraries. One notable example is in the
drawing of two-dimensional graphics. In this section we'll discuss a class that uses
inheritance to draw complex two-dimensional shapes and to assign colors to them.
In the Chapter 3 supplement on graphics, we introduced an object called
Graphics which acts like a pen that you can use to draw shapes and lines onto a win-
dow. When Java's designers wanted additional graphical functionality, they extended
the Graphics class into a more powerful class called Graphics2D . This is a good
example of one of the more common uses of inheritance: to extend and reuse func-
tionality from a powerful existing object.
Why didn't Sun simply add the new methods into the existing Graphics class?
The Graphics class already worked properly, so Sun decided it was best not to per-
form unnecessary surgery on it. John Vlissides, part of a famous foursome of soft-
ware engineers affectionately called the “Gang of Four,” once described the idea this
way: “A hallmark—if not the hallmark—of good object-oriented design is that you
can modify and extend a system by adding code rather than by hacking it. In short,
change is additive, not invasive.”
Making Graphics2D extend Graphics retains backward compatibility. Backward
compatibility is the ability of new code to work correctly with old code without mod-
ifying the old code. Leaving Graphics untouched ensured that old programs would
keep working properly and gave new programs the option to use the new
Graphics2D functionality.
Sun's documentation for Graphics2D describes the purpose of the class as fol-
lows. “This Graphics2D class extends the Graphics class to provide more sophisti-
cated control over geometry, coordinate transformations, color management, and text
layout. This is the fundamental class for rendering 2-dimensional shapes, text, and
images on the Java™ platform.” To be specific, Graphics2D adds the ability to per-
form transformations such as scaling and rotation when you're drawing. These capa-
bilities can lead to some fun and interesting images on the screen.
If you used the DrawingPanel class from Chapter 3's graphical supplement, you
previously wrote statements like the following to get access to the panel's Graphics
object:
Graphics g = panel.getGraphics();
Actually, the getGraphics method doesn't return a Graphics object at all, but
rather a Graphics2D object. Because of polymorphism, though, it is legal for your
program to treat it as a Graphics object, because every Graphics2D object “is” a
Graphics object. To use it as a Graphics2D object instead, simply write the follow-
ing line of code:
Graphics2D g2 = panel.getGraphics();