RECURSION - Introduction to Java Programming - page 755

How do you draw a Sierpinski triangle of order 1 ? The problem can be reduced to drawing
three Sierpinski triangles of order 0 . How do you draw a Sierpinski triangle of order 2 ? The
problem can be reduced to drawing three Sierpinski triangles of order 1 , so the problem of
drawing a Sierpinski triangle of order n can be reduced to drawing three Sierpinski triangles
of order n - 1 .
Listing 20.9 gives a Java applet that displays a Sierpinski triangle of any order, as shown in
Figure 20.9. You can enter an order in a text field to display a Sierpinski triangle of the spec-
ified order.

L ISTING 20.9 SierpinskiTriangle.java
1 import javax.swing.*;
2 import java.awt.*;
3 import java.awt.event.*;
4
5 public class SierpinskiTriangle extends JApplet {
6 private JTextField jtfOrder = new JTextField( "0" , 5 ); // Order

7 private SierpinskiTrianglePanel trianglePanel =

8 new SierpinskiTrianglePanel(); // To display the pattern

9
10 public SierpinskiTriangle() {
11 // Panel to hold label, text field, and a button
12 JPanel panel = new JPanel();
13 panel.add( new JLabel( "Enter an order: " ));
14 panel.add(jtfOrder);
15 jtfOrder.setHorizontalAlignment(SwingConstants.RIGHT);
16
17 // Add a Sierpinski triangle panel to the applet
18 add(trianglePanel);
19 add(panel, BorderLayout.SOUTH);
20
21

// Register a listener

22
23 @Override
24 public void actionPerformed(ActionEvent e) {
25 trianglePanel.setOrder(Integer.parseInt(jtfOrder.getText()));
26 }
27 });
28 }
29
30

jtfOrder.addActionListener( new ActionListener() {

listener

set a new order

static class SierpinskiTrianglePanel extends JPanel {

31 private int order = 0 ;

32
33 /** Set a new order */
34 public void setOrder( int order) {
35 this .order = order;
36 repaint();
37 }
38
39 @Override
40 protected void paintComponent(Graphics g) {

41 super .paintComponent(g);

42
43 // Select three points in proportion to the panel size

44
45
46
47

Point p1 = new Point(getWidth() / 2 , 10 ); three initial points

Point p2 = new Point( 10 , getHeight() - 10 );

Point p3 = new Point(getWidth() - 10 , getHeight() - 10 );