590 Chapter 17 Creating Graphical User Interfaces

17.8 Combo Boxes

A combo box, also known as a choice list or drop-down list, contains a list of items from which the user can choose. It is useful in limiting a user’s range of choices and avoids the cumbersome validation of data input. Figure 17.21 lists several frequently used constructors and methods in JComboBox.

FIGURE 17.21 JComboBox enables you to select an item from a set of items.

The following statements create a combo box with four items, red foreground, white background, and the second item selected.

JComboBox jcb = new JComboBox(new Object[] {"Item 1", "Item 2", "Item 3", "Item 4"});

jcb.setForeground(Color.red);

jcb.setBackground(Color.white); jcb.setSelectedItem("Item 2");

JComboBox can fire ActionEvent and ItemEvent, among many other events. Whenever an item is selected, an ActionEvent is fired. Whenever a new item is selected, JComboBox fires ItemEvent twice, once for deselecting the previously selected item, and the other for selecting the currently selected item. Note that no ItemEvent is fired if the current item is reselected. To respond to an ItemEvent, you need to implement the itemStateChanged(ItemEvent e) handler for processing a choice. To get data from a JComboBox menu, you can use getSelectedItem() to return the currently selected item, or e.getItem() method to get the item from the itemStateChanged(ItemEvent e) handler.

Listing 17.8 gives a program that lets users view an image and a description of a country’s flag by selecting the country from a combo box, as shown in Figure 17.22.

Here are the major steps in the program:

1.Create the user interface.

Create a combo box with country names as its selection values. Create a DescriptionPanel object. The DescriptionPanel class was introduced in the preceding

The listener listens to ItemEvent from the combo box and implements ItemListener (lines 70–75). Instead of using ItemEvent, you may rewrite the program to use ActionEvent for handling combo-box item selection.

The program stores the flag information in three arrays: flagTitles, flagImage, and flagDescription (lines 7–25). The array flagTitles contains the names of nine countries, the array flagImage contains images of the nine countries’ flags, and the array flagDescription contains descriptions of the flags.

17.9 Lists 593

The program creates an instance of DescriptionPanel (line 28), which was presented in Listing 17.6, DescriptionPanel.java. The program creates a combo box with initial values from flagTitles (line 31). When the user selects an item in the combo box, the itemStateChanged handler is executed, finds the selected index, and sets its corresponding flag title, flag image, and flag description on the panel.

17.9 Lists

A list is a component that basically performs the same function as a combo box but enables the user to choose a single value or multiple values. The Swing JList is very versatile. Figure 17.23 lists several frequently used constructors and methods in JList.

javax.swing.JComponent

The get and set methods for these data fields are provided in the class, but omitted in the UML diagram for brevity.

javax.swing.JList

-selectedIndex: int -selectedIndices: int[] -selectedValue: Object -visibleRowCount: int

-selectionBackground: Color -selectionForeground: Color -selectionMode: int

+JList()

+JList(items: Object[])

+addListSelectionListener(listener: ListSelectionListener): void

FIGURE 17.23 JList enables you to select multiple items from a set of items.

selectionMode is one of the three values (SINGLE_SELECTION, SINGLE_INTERVAL _SELECTION, MULTIPLE_INTERVAL_SELECTION) defined in javax.swing.ListSelectionModel that indicate whether a single item, single-interval item, or multiple-interval item can be selected. Single selection allows only one item to be selected. Single-interval selection allows multiple selections, but the selected items must be contiguous. Multipleinterval selection allows selections of multiple contiguous items without restrictions, as shown in Figure 17.24. The default value is MULTIPLE_INTERVAL_SELECTION.

FIGURE 17.24 JList has three selection modes: single selection, single-interval selection, and multiple-interval selection.

17.9 Lists 595

10"France", "Germany", "India", "Norway", "United Kingdom",

11"United States of America"};

12

13// The list for selecting countries

14private JList jlst = new JList(flagTitles);

15

16// Declare an ImageIcon array for the national flags of 9 countries

17private ImageIcon[] imageIcons = {

18new ImageIcon("image/ca.gif"),

19new ImageIcon("image/china.gif"),

20new ImageIcon("image/denmark.gif"),

21new ImageIcon("image/fr.gif"),

22new ImageIcon("image/germany.gif"),

23new ImageIcon("image/india.gif"),

24new ImageIcon("image/norway.gif"),

25new ImageIcon("image/uk.gif"),

26new ImageIcon("image/us.gif")

27};

28

29// Arrays of labels for displaying images

30private JLabel[] jlblImageViewer = new JLabel[NUMBER_OF_FLAGS];

32public static void main(String[] args) {

34frame.setSize(650, 500);

35frame.setTitle("ListDemo");

36frame.setLocationRelativeTo(null); // Center the frame

37frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

38frame.setVisible(true);

39}

40

41public ListDemo() {

42// Create a panel to hold nine labels

45for (int i = 0; i < NUMBER_OF_FLAGS; i++) {

46p.add(jlblImageViewer[i] = new JLabel());

47jlblImageViewer[i].setHorizontalAlignment

48(SwingConstants.CENTER);

49}

50

51// Add p and the list to the frame

52add(p, BorderLayout.CENTER);

53add(new JScrollPane(jlst), BorderLayout.WEST);

55// Register listeners

56jlst.addListSelectionListener(new ListSelectionListener() {

57/** Handle list selection */

59// Get selected indices

60int[] indices = jlst.getSelectedIndices();

62int i;

63// Set icons in the labels

64for (i = 0; i < indices.length; i++) {

65jlblImageViewer[i].setIcon(imageIcons[indices[i]]);

66}

67

68// Remove icons from the rest of the labels

69for (; i < NUMBER_OF_FLAGS; i++) {