Learning Java, 4th Edition

The Big Picture

The classes you’ll use for drawing come from six packages: java.awt, java.awt.color, java.awt.font, java.awt.geom, java.awt.image, and java.awt.print. Collectively, these classes make up most of the 2D API and cover the drawing of shapes, text, and images. Figure 20-1 shows a bird’s-eye view of these classes. There’s much more in the 2D API than we can cover in two chapters. For a full treatment, see Jonathan Knudsen’s Java 2D Graphics (O’Reilly).

An instance of java.awt.Graphics2D is called a graphics context. It represents a drawing surface—such as a component’s display area, a page on a printer, or an offscreen image buffer. A graphics context provides methods for drawing three kinds of graphics objects: shapes, text, and images. Graphics2D is called a graphics context because it also holds contextual information about the drawing area. This information includes the drawing area’s clipping region, painting color, transfer mode, text font, and geometric transformation. If you consider the drawing area to be a painter’s canvas, you might think of a graphics context as an easel that holds a set of tools and marks off the work area.

Figure 20-1. Graphics classes of the 2D API

There are four ways to acquire a Graphics2D object. The following list describes them in order from the most common to the least:

From AWT or Swing as the result of a painting request on a component

In this case, a new graphics context for the appropriate area is created and passed to your component’s paint() or update() method. (The update() method really applies only to AWT components, not the newer Swing components.)

Directly from an offscreen image buffer

In this case, we ask the image buffer for a graphics context directly. We’ll use this when we discuss techniques such as double buffering.

By copying an existing Graphics2D object

Duplicating a graphics object can be useful for more elaborate drawing operations; different copies of a Graphics2D object can draw on the same area, but with different attributes and clipping regions. A Graphics2D object can be copied by calling the create() method.

Directly from an onscreen component

It’s possible to ask a component to give you a Graphics2D object for its display area. However, this is almost always a mistake; if you feel tempted to do this, think about why you’re trying to circumvent the normal paint()/repaint() mechanism.

Each time a component’s paint() method is called, the windowing system provides the component with a new Graphics2D object for drawing in the display area. This means that attributes set during one painting session, such as the drawing color or clipping region, are reset the next time paint() is called. (Each call to paint() starts with a tidy new easel.) For the most common attributes, such as foreground color, background color, and font, we can set defaults in the component itself. Thereafter, the graphics contexts for painting in that component come with those properties initialized appropriately.

The paint() method can make no assumptions about what is already drawn on the screen. It is responsible for rendering its entire work area. Higher-level APIs are normally responsible for buffering output and limiting the number of times paint() is invoked for a component. AWT components may use an additional method called update(), which allows them to update their appearance under the assumption that their previous artwork is still on the screen. However, this method is not used by Swing components.

For backward compatibility, a graphics context is always passed to the paint() method as an object of type Graphics. If you want to take advantage of the nifty features in the 2D API (as you almost undoubtedly will), you need to cast this reference to a Graphics2D object. You’ll see how this works in the upcoming examples.