17.1 Introduction

17.2 Buttons 573

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.AbstractButton

-actionCommand: String -text: String

-icon: javax.swing.Icon

-pressedIcon: javax.swing.Icon -rolloverIcon: javax.swing.Icon -mnemonic: int

-horizontalAlignment: int -horizontalTextPosition: int -verticalAlignment: int -verticalTextPosition: int -borderPainted: boolean

-iconTextGap: int -selected(): boolean

FIGURE 17.2 AbstractButton defines common features of different types of buttons.

javax.swing.AbstractButton

javax.swing.JButton

+JButton()

+JButton(icon: javax.swing.Icon) +JButton(text: String) +JButton(text: String, icon: Icon)

+addActionListener(listener: ActionListener) : void

FIGURE 17.3 JButton defines a regular push button.

For example, Listing 17.1 displays the American flag as a regular icon, the Canadian flag as a pressed icon, and the British flag as a rollover icon, as shown in Figure 17.4.

LISTING 17.1 TestButtonIcons.java

6JFrame frame = new TestButtonIcons();

7 frame.setTitle("ButtonIcons");

8frame.setSize(200, 100);

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

10 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

576Chapter 17 Creating Graphical User Interfaces

2.Process the event.

Create and register listeners for processing the action event to move the message left or right according to whether the left or right button was clicked.

MessagePanel

JPanel with

FlowLayout

FIGURE 17.9 Clicking the <= and => buttons causes the message on the panel to move to the left and right, respectively.

LISTING 17.2 ButtonDemo.java

47// Place panels in the frame

48setLayout(new BorderLayout());

49add(messagePanel, BorderLayout.CENTER);

50add(jpButtons, BorderLayout.SOUTH);

54public void actionPerformed(ActionEvent e) {

55messagePanel.moveLeft();

56}

57});

59public void actionPerformed(ActionEvent e) {

60messagePanel.moveRight();

61}

62});

63}

64}

messagePanel (line 8) is deliberately declared protected so that it can be referenced by a subclass in future examples.

You can set an icon image on the button by using the setIcon method. If you uncomment the following code in lines 38–41:

//jbtLeft.setIcon(new ImageIcon("image/left.gif"));

//jbtRight.setIcon(new ImageIcon("image/right.gif"));

//jbtLeft.setText(null);

//jbtRight.setText(null);

the texts are replaced by the icons, as shown in Figure 17.10(a). "image/left.gif" is located in "c:\book\image\left.gif". Note that the backslash is the Windows file-path notation. In Java, the forward slash should be used.

FIGURE 17.10 You can set an icon on a JButton and access a button using its mnemonic key.

You can set text and an icon on a button at the same time, if you wish, as shown in Figure 17.10(b). By default, the text and icon are centered horizontally and vertically.

The button can also be accessed by using the keyboard mnemonics. Pressing Alt+ L is equivalent to clicking the <= button, since you set the mnemonic property to 'L' in the left button (line 34). If you change the left button text to "Left" and the right button text to "Right", the L and R in the captions of these buttons will be underlined, as shown in Figure 17.10(b).