GRAPHICS - Introduction to Java Programming - page 482

4 public class TestPaintComponent extends JFrame {
5 public TestPaintComponent() {
6 add(

new NewPanel() );

create a panel

7 }
8
9 public static void main(String[] args) {
10 TestPaintComponent frame = new TestPaintComponent();
11 frame.setTitle( "TestPaintComponent" );
12 frame.setSize( 200 , 100 );
13 frame.setLocationRelativeTo( null ); // Center the frame
14 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
15 frame.setVisible( true );
16 }
17 }
18
19
20
21
22
23
24
25
26 }

class NewPanel extends JPanel { new panel class

@Override

override paintComponent
draw things in the superclass
draw line
draw string

protected void paintComponent(Graphics g) {

super .paintComponent(g);

g.drawLine( 0 , 0 , 50 , 50 );

g.drawString( "Banner" , 0 , 40 );

}

(0, 0)

(0, 40)

This is a JPanel
object placed
inside a frame

(50, 50)

F IGURE 13.4

A line and a string are drawn on a panel.

The paintComponent method is automatically invoked to paint graphics when the com-
ponent is first displayed or whenever the component needs to be redisplayed. Invoking
super.paintComponent(g) (line 22) invokes the paintComponent method defined in
the superclass. This is necessary to ensure that the viewing area is cleared before a new draw-
ing is displayed. Line 23 invokes the drawLine method to draw a line from ( 0 , 0 ) to ( 50 , 50 ).
Line 24 invokes the drawString method to draw the string Banner at location ( 0 , 40 ).
All the drawing methods have parameters that specify the locations of the subjects to be
drawn. In Java, all measurements are made in pixels.
The JVM invokes paintComponent to draw things on a component. The user should
never invoke paintComponent directly. For this reason, the protected visibility is sufficient
for paintComponent .
Panels are invisible and are used as small containers that group components to achieve a
desired layout. Another important use of JPanel is for drawing. You can draw things on any
Swing GUI component, but normally you should use a JPanel as a canvas upon which to
draw things. What happens if you replace JPanel with JLabel in line 19 as follows?

class NewPanel extends JLabel {

The program will work, but it is not preferred, because JLabel is designed for creating a
label, not for drawing. For consistency, this topic will define a canvas class by subclassing
JPanel .

extends JPanel ?