GRAPHICS - Introduction to Java Programming - page 480

13.1 Introduction
You can draw custom shapes on a GUI component.

Key
Point

Suppose you want to draw shapes such as a bar chart, a clock, or a stop sign, as shown in
Figure 13.1. How do you do so?

Problem

(a) (b) (c)

F IGURE 13.1

You can draw shapes using the drawing methods in the Graphics class.

This chapter describes how to use the methods in the Graphics class to draw strings,
lines, rectangles, ovals, arcs, polygons, and images, and how to develop reusable GUI compo-
nents.

13.2 The Graphics Class
Each GUI component has a graphics context, which is an object of the Graphics
class. The Graphics class contains the methods for drawing various shapes.

Key
Point

The Graphics class provides the methods for drawing strings, lines, rectangles, ovals, arcs,
polygons, and polylines, as shown in Figure 13.2.
Think of a GUI component as a piece of paper and the Graphics object as a pencil or
paintbrush. You can apply the methods in the Graphics class to draw graphics on a GUI
component.
To paint, you need to specify where to paint. Each component has its own coordinate sys-
tem with the origin ( 0 , 0 ) at the upper-left corner. The x- coordinate increases to the right, and
the y -coordinate increases downward. Note that the Java coordinate system differs from the
conventional coordinate system, as shown in Figure 13.3.
The Graphics class-an abstract class—provides a device-independent graphics interface
for displaying figures and images on the screen on different platforms. Whenever a compo-
nent (e.g., a button, a label, or a panel) is displayed, the JVM automatically creates a
Graphics object for the component on the native platform and passes this object to invoke
the paintComponent method to display the drawings.
The signature of the paintComponent method is as follows: paintComponent

protected void paintComponent(Graphics g)

This method, defined in the JComponent class, is invoked whenever a component is first dis-
played or redisplayed.
To draw on a component, you need to define a class that extends JPanel and overrides its
paintComponent method to specify what to draw. Listing 13.1 gives an example that draws
a line and a string on a panel, as shown in Figure 13.4.