GRAPHICS - Introduction to Java Programming - page 509

2.

Whenever a component (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.

3.

Normally you use JPanel as a canvas. To draw on a JPanel , you create a new class
that extends JPanel and overrides the paintComponent method to tell the panel
how to draw graphics.

4.

Invoking super.paintComponent(g) is necessary to ensure that the viewing area
is cleared before a new drawing is displayed. The user can request the component to
be redisplayed by invoking the repaint() method defined in the Component class.
Invoking repaint() causes paintComponent to be invoked by the JVM. The user
should never invoke paintComponent directly. For this reason, the protected visi-
bility is sufficient for paintComponent .

5.

The Component class has the setBackground , setForeground , and setFont
methods. These methods are used to set colors and fonts for the entire component. If
you want to draw several messages in a panel with different colors and fonts, you
have to use the setColor and setFont methods in the Graphics class to set the
color and font for the current drawing.

6. FontMetrics can be used to compute the exact length and width of a string,
which is helpful for measuring the size of a string in order to display it in the right
position.

7.

To display an image, first create an image icon. You can then use ImageIcon 's
getImage() method to get an Image object for the image and draw the image using
the drawImage method in the java.awt.Graphics class.

T EST Q UESTIONS

Do the test questions for this chapter online at www.cs.armstrong.edu/liang/intro9e/test.html .

P ROGRAMMING E XERCISES

Sections 13.2-13.7
*13.1

( Display a grid ) Write a program that displays a grid, as shown in
Figure 13.26a. Use red color for vertical lines and blue for horizontals.

3 * 3 3 * 3

**13.2

( Create a custom button class ) Develop a custom button class named
OvalButton that extends JButton and displays the button text inside an oval.
Figure 13.26b shows two buttons created using the OvalButton class.

*13.3

( Display a checkerboard ) Programming Exercise 12.10 displays a checkerboard
in which each white and black cell is a JButton . Rewrite a program that draws a
checkerboard on a JPanel using the drawing methods in the Graphics class, as
shown in Figure 13.26c. Use the drawRect method to draw each cell in the
checkerboard.

*13.4

( Display a multiplication table ) Write a program that displays a multiplication
table in a panel using the drawing methods, as shown in Figure 13.27a.