Java Never Ends - Absolute Java - page 1082

“Wait a minute,” you may think, “the program in Display 19.1 was not supposed
to use threads in any essential way.” That is basically true, but every Java program uses
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.)
The method Thread.sleep can sometimes be handy even if you do not do any
multithreaded programming. The class Thread is in the package java.lang and so
requires no import statement.

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 }