Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 852

29 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
30 frame.setSize( new Dimension(350, 100));
31 frame.setLayout( new FlowLayout());
32 frame.add(numberField);
33 frame.add(verifyButton);
34 frame.add(validLabel);
35 frame.setVisible( true );
36 }
37
38 // Returns whether the given string is a valid Visa
39 // card number according to the Luhn checksum algorithm.
40 public boolean isValidCC(String text) {
41 int sum = 0;
42 for ( int i = text.length() - 1; i >= 0; i--) {
43 int digit = Integer.parseInt(
44 text.substring(i, i + 1));
45 if (i % 2 == 0) { // double even digits
46 digit *= 2;
47 }
48 sum += (digit / 10) + (digit % 10);
49 }
50
51 // valid numbers add up to a multiple of 10
52 return sum % 10 == 0 && text.startsWith("4");
53 }
54
55 // Sets label's text to show whether CC number is valid.
56 public void actionPerformed(ActionEvent event) {
57 String text = numberField.getText();
58 if (isValidCC(text)) {
59 validLabel.setText("Valid number!");
60 } else {
61 validLabel.setText("Invalid number.");
62 }
63 }
64 }
14.4 Additional Components and Events
The GUIs we have developed so far use only a few components (buttons, text fields,
and labels) and can respond to only one kind of event (action events). In this section,
we'll look at some other useful components and events that GUI programs can use
and to which they can respond.