Java How to Program Fourth Edition - DCC

Chapter 13 Graphical User Interface Components: Part 2 727
The program of Fig. 13.2 and Fig. 13.3 demonstrates a customized subclass of
JPanel. Class CustomPanel (Fig. 13.2) has its own paintComponent method that
draws a circle or a square, depending on the value passed to CustomPanel’s draw
method. For this purpose, CustomPanel line 11 defines constants that enable the program
to specify the shape a CustomPanel draws on itself with each call to its paint-
Component method. Class CustomPanelTest (Fig. 13.3) creates a CustomPanel
and a GUI that enable the user to choose which shape to draw.
Class CustomPanel contains one instance variable, shape, that stores an integer
representing the shape to draw. Method paintComponent (lines 15–23) draws a shape
on the panel. If shape is CIRCLE, Graphics method fillOval draws a solid circle.
If shape is SQUARE, Graphics method fillRect draws a solid square. Method
draw (lines 26–30) sets instance variable shape and calls repaint to refresh the CustomPanel
object. Note that calling repaint (which is really this.repaint()) for
the CustomPanel schedules a painting operation for the CustomPanel. Method
paintComponent will be called to repaint the CustomPanel and draw the appropriate
shape.
1 // Fig. 13.2: CustomPanel.java
2 // A customized JPanel class.
3
4 // Java core packages
5 import java.awt.*;
6
7 // Java extension packages
8 import javax.swing.*;
9
10 public class CustomPanel extends JPanel {
11 public final static int CIRCLE = 1, SQUARE = 2;
12 private int shape;
13
14 // use shape to draw an oval or rectangle
15 public void paintComponent( Graphics g )
16 {
17 super.paintComponent( g );
18
19 if ( shape == CIRCLE )
20 g.fillOval( 50, 10, 60, 60 );
21 else if ( shape == SQUARE )
22 g.fillRect( 50, 10, 60, 60 );
23 }
24
25 // set shape value and repaint CustomPanel
26 public void draw( int shapeToDraw )
27 {
28 shape = shapeToDraw;
29 repaint();
30 }
31
32 } // end class CustomPanel
Fig. Fig. 13.2 13.2 Defining a custom drawing area by subclassing JPanel.