Before we go any further, one concept we need to understand is that of a graphics context,
which in Java is represented by the Graphics class.
What is a graphics context?
Basically, it provides a context within which you can draw. As the Graphics
class documents (see the Java API javadocs), it encapsulates state information needed for the basic
rendering operations that Java supports. This state information includes the
following properties:
- The
Component object on which to draw.
- A translation origin for rendering and clipping coordinates.
- The current clip.
- The current color.
- The current font.
- The current logical pixel operation function (XOR or Paint).
- The current XOR alternation color (see later).
We will see how you can draw inside Java Swing frames, panels, and other GUI
components. If you ever venture into printing from Java, you will find the
same concept there, except that the context is focused not on a GUI window, but
on a printable page. We won't get into printing from Java in this course
-- that's a more advanced topic.
For drawing on the screen, Java originally provided only the Graphics class, which is
actually an abstract base class, from which all graphics context classes
extend. You can see what it (or rather, a
class that extends it) provides by going to its javadoc
page.
Java 1.2 and beyond adds the Java 2D library, which augments the ability to
draw to a considerable degree. It uses Graphics2, which is
another abstract class that extends the Graphics class. You
can find the details at its javadoc
page.
Graphics2 provides much more sophisticated control over geometry,
coordinate transformations, color management, and text layout. Thoroughly
covering the Java 2D library could take up an entire semester course, so we
won't be going into detail on Java 2D. We may show some examples of Java
2D later.
We'll be seeing the use of a graphics context in lots of subsequent drawing
examples.
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