25 add(jpRadioButtons, BorderLayout.WEST);

26

27// Create a radio-button group to group three buttons

28ButtonGroup group = new ButtonGroup();

29group.add(jrbRed);

30group.add(jrbGreen);

31group.add(jrbBlue);

32

33// Set keyboard mnemonics

34jrbRed.setMnemonic('E');

35jrbGreen.setMnemonic('G');

36jrbBlue.setMnemonic('U');

38// Register listeners for radio buttons

39jrbRed.addActionListener(new ActionListener() {

40public void actionPerformed(ActionEvent e) {

41messagePanel.setForeground(Color.red);

42}

43});

44jrbGreen.addActionListener(new ActionListener() {

45public void actionPerformed(ActionEvent e) {

46messagePanel.setForeground(Color.green);

47}

48});

49jrbBlue.addActionListener(new ActionListener() {

50public void actionPerformed(ActionEvent e) {

51messagePanel.setForeground(Color.blue);

52}

53});

54

55// Set initial message color to blue

56jrbBlue.setSelected(true);

57messagePanel.setForeground(Color.blue);

58}

59}

RadioButtonDemo extends CheckBoxDemo and adds three radio buttons to specify the message color. When a radio button is clicked, its action event listener sets the corresponding foreground color in messagePanel.

The keyboard mnemonics 'R' and 'B' are already set for the Right button and Bold check box. To avoid conflict, the keyboard mnemonics 'E', 'G', and 'U' are set on the radio buttons Red, Green, and Blue, respectively (lines 34–36).

The program creates a ButtonGroup and puts three JRadioButton instances (jrbRed, jrbGreen, and jrbBlue) in the group (lines 28–31).

A radio button fires an ActionEvent and an ItemEvent when it is selected or deselected. You could process either the ActionEvent or the ItemEvent to choose a color. The example processes the ActionEvent. Please rewrite the code using the ItemEvent as an exercise.

17.5 Labels

A label is a display area for a short text, an image, or both. It is often used to label other components (usually text fields). Figure 17.15 lists the constructors and methods in JLabel.

JLabel inherits all the properties from JComponent and has many properties similar to the ones in JButton, such as text, icon, horizontalAlignment, verticalAlignment,

17.5 Labels 583

group buttons

register listener

register listener

register listener

Video Note

Use labels and text fields

17.6 Text Fields

A text field can be used to enter or display a string. JTextField is a subclass of JTextComponent. Figure 17.16 lists the constructors and methods in JTextField.

JTextField inherits JTextComponent, which inherits JComponent. Here is an example of creating a text field with red foreground color and right horizontal alignment:

JTextField jtfMessage = new JTextField("T-Strom"); jtfMessage.setForeground(Color.RED); jtfMessage.setHorizontalAlignment(SwingConstants.RIGHT);

javax.swing.text.JTextComponent

-text: String -editable: boolean

javax.swing.JTextField

-columns: int -horizontalAlignment: int

+JTextField() +JTextField(column: int) +JTextField(text: String)

+JTextField(text: String, columns: int)

+addActionEvent(listener: ActionListener): void

17.6 Text Fields 585

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

The text contained in this text component.

Indicates whether this text component is editable (default: true).

The number of columns in this text field.

The horizontal alignment of this text field (default: LEFT). Creates a default empty text field with number of columns set to 0. Creates an empty text field with a specified number of columns. Creates a text field initialized with the specified text.

Creates a text field initialized with the specified text and columns. Adds an ActionListener for this object.

FIGURE 17.17 A label and a text field are added to set a new message.

LISTING 17.5 TextFieldDemo.java

1 import java.awt.*;

2 import java.awt.event.*;

3 import javax.swing.*;

4

5 public class TextFieldDemo extends RadioButtonDemo {

6 private JTextField jtfMessage = new JTextField(10);

7

8/** Main method */

9public static void main(String[] args) {

10TextFieldDemo frame = new TextFieldDemo();

11frame.pack();

12frame.setTitle("TextFieldDemo");

13frame.setLocationRelativeTo(null); // Center the frame

14frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

15frame.setVisible(true);

16}

17

18public TextFieldDemo() {

19// Create a new panel to hold label and text field

20JPanel jpTextField = new JPanel();

21jpTextField.setLayout(new BorderLayout(5, 0));

ButtonDemo

CheckBoxDemo

RadioButtonDemo

TextFieldDemo

create frame pack frame

create UI

A label showing an image and a text

DescriptionPanel

with BorderLayout

A text area inside a scroll panel