Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 865

moving picture. A timer is an object that, once started, fires an action event at regular
intervals. You supply the timer with an ActionListener to use and a delay time in
milliseconds between action events, and it does the rest. You need to include two
objects to create an animation:
• An ActionListener object that updates the way the panel draws itself
•A Timer object to invoke the ActionListener object at regular intervals, caus-
ing the panel to animate
Let's modify our RectPanel class from the previous section to move its rectan-
gles on the panel. To make them move, we must store their positions as fields in the
JPanel object. We'll repeatedly change the fields' values and redraw the rectangles
as the program is running, which will make it look like they're moving. We'll store
the desired change in x and change in y as fields called dx and dy , respectively.
Here's the code for our animated RectPanel :
1 // A panel that draws animated rectangles on its surface.
2 // Initial version-will change in the following section.
3
4 import java.awt.*;
5 import java.awt.event.*;
6 import javax.swing.*;
7
8 public class AnimatedRectPanel1 extends JPanel {
9 private Point p1; // location of first rectangle
10 private Point p2; // location of second rectangle
11 private int dx; // amount by which to move horizontally
12 private int dy; // amount by which to move vertically
13
14 public AnimatedRectPanel1() {
15 p1 = new Point(20, 40);
16 p2 = new Point(60, 10);
17 dx = 5;
18 dy = 5;
19 }
20
21 // draws the panel on the screen
22 public void paintComponent(Graphics g) {
23 super .paintComponent(g); // call JPanel's version
24 g.setColor(Color.RED);
25 g.fillRect(p1.x, p1.y, 70, 30);
26 g.setColor(Color.BLUE);
27 g.fillRect(p2.x, p2.y, 20, 80);
28 }
29 }