Java Never Ends - Java - page 1028

threads in some way. If there is only one stream of computation, as in Display 19.1,
then that is treated as a single thread by Java. So, threads are always used by Java, but
not in an interesting way until more than one thread is used.
You can safely think of the invocation of

Thread.sleep(milliseconds);

as a pause in the computation that lasts (approximately) the number of milliseconds given
as the argument. (If this invocation is in a thread of a multithreaded program, then the
pause, like anything else in the thread, applies only to the thread in which it occurs.)

Display 19.1 Nonresponsive GUI (part 1 of 3)

1 import javax.swing.JFrame;

2 import javax.swing.JPanel;

3 import javax.swing.JButton;

4 import java.awt.BorderLayout;

5 import java.awt.FlowLayout;

6 import java.awt.Graphics;

7 import java.awt.event.ActionListener;

8 import java.awt.event.ActionEvent;

9 /**

10 Packs a section of the frame window with circles, one at a time.

11 */

12 public class FillDemo extends JFrame implements ActionListener

13 {

14 public static final int WIDTH = 300;

15 public static final int HEIGHT = 200;

16 public static final int FILL_WIDTH = 300;

17 public static final int FILL_HEIGHT = 100;

18 public static final int CIRCLE_SIZE = 10;

19 public static final int PAUSE = 100; //milliseconds

20 private JPanel box;

21 public static void main(String[] args)

22 {

23 FillDemo gui = new FillDemo();

24 gui.setVisible(true);

25 }

26 public FillDemo()

27 {

28 setSize(WIDTH, HEIGHT);

29 setTitle("FillDemo");

30 setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);