16.2. The Applet Class
15.2. Buttons
A button is a component that triggers an action event when clicked. Swing provides regular buttons, toggle buttons, check box buttons, and radio buttons. The common features of these buttons are represented in javax.swing.AbstractButton , as shown in Figure 15.2.
Figure 15.2. AbstractButton defines common features of different types of buttons.
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This section introduces the regular buttons defined in the JButton class. JButton inherits AbstractButton and provides several constructors to create buttons, as shown in Figure 15.3.
Figure 15.3. JButton defines a regular push button.
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15.2.1. Icons, Pressed Icons, and Rollover Icons
A regular button has a default icon, pressed icon, and rollover icon. Normally, you use the default icon. The other icons are for special effects. A pressed icon is displayed when a button is pressed, and a rollover icon is displayed when the mouse is over the button but not pressed. For example, Listing 15.1 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 15.4.
Figure 15.4. A button can have several types of icons.
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Listing 15.1. TestButtonIcons.java
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15.2.2. Alignments
Horizontal alignment specifies how the icon and text are placed horizontally on a button. You can set the horizontal alignment using setHorizontalAlignment(int) with one of the five constants LEADING , LEFT , CENTER , RIGHT , TRAILING , as shown in Figure 15.5. At present, LEADING and LEFT are the same, and TRAILING and RIGHT are the same. Future implementation may distinquish them. The default horizontal alignment is SwingConstants.TRAILING .
Figure 15.5. You can specify how the icon and text are placed on a button horizontally.
Vertical alignment specifies how the icon and text are placed vertically on a button. You can set the vertical alignment using setVerticalAlignment(int) with one of the three constants TOP , CENTER , BOTTOM , as shown in Figure 15.6. The default vertical alignment is SwingConstants.CENTER .
Figure 15.6. You can specify how the icon and text are placed on a button vertically.
15.2.3. Text Positions
Horizontal text position specifies the horizontal position of the text relative to the icon. You can set the horizontal text position using setHorizontalTextPosition(int) with one of the five constants LEADING , LEFT , CENTER , RIGHT , TRAILING , as shown in Figure 15.7. At present, LEADING and LEFT are the same, and TRAILING and RIGHT are the same. Future implementation may distinquish them. The default horizontal text position is SwingConstants.RIGHT .
Figure 15.7. You can specify the horizontal position of the text relative to the icon.
Vertical text position specifies the vertical position of the text relative to the icon. You can set the vertical text position using setVerticalTextPosition(int) with one of the three constants TOP , CENTER , BOTTOM , as shown in Figure 15.8. The default vertical text position is SwingConstants.CENTER .
Figure 15.8. You can specify the vertical position of the text relative to the icon.
Note
| | The constants LEFT , CENTER , RIGHT , LEADING , TRAILING , TOP , and BOTTOM used in AbstractButton are also used in many other Swing components . These constants are centrally defined in the javax.swing.SwingConstants interface. Since all Swing GUI components implement SwingConstants , you can reference the constants through SwingConstants or a GUI component. For example, SwingConstants.CENTER is the same as JButton.CENTER . |
JButton can generate many types of events, but often you need to respond to an ActionEvent . When a button is pressed, it generates an ActionEvent .
15.2.4. Example: Using Buttons
This example presents a program, shown in Listing 15.2, that displays a message on a panel and uses two buttons, <= and => , to move the message on the panel to the left or right. The layout of the UI and the output of the program are shown in Figure 15.9.
Figure 15.9. Clicking the <= and => buttons causes the message on the panel to move to the left and right, respectively.
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Here are the major steps in the program:
| 1. | Create the user interface. Create a MessagePanel object to display the message. The MessagePanel class was created in §13.11, "Case Study: The MessagePanel Class." Place it in the center of the frame. Create two buttons, <= and => , on a panel. Place the panel in the south of the frame. |
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| 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. |
Listing 15.2. ButtonDemo.java
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1 import java.awt.*; 2 import java.awt.event.ActionListener; 3 import java.awt.event.ActionEvent; 4 import javax.swing.*; 5 6 public class ButtonDemo extends JFrame { 7 // Create a panel for displaying message 8 protected MessagePanel messagePanel 9 = new MessagePanel( "Welcome to Java" ); 10 11 // Declare two buttons to move the message left and right 12 private JButton jbtLeft = new JButton( "<=" ); 13 private JButton jbtRight = new JButton( "=>" ); 14 15 public static void main(String[] args) { 16 ButtonDemo frame = new ButtonDemo(); 17 frame.setTitle( "ButtonDemo" ); 18 frame.setLocationRelativeTo( null ); // Center the frame 19 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 20 frame.setSize( 250 , 100 ); 21 frame.setVisible( true ); 22 } 23 24 public ButtonDemo() { 25 // Set the background color of messagePanel 26 messagePanel.setBackground(Color.white); 27 28 // Create Panel jpButtons to hold two Buttons "<=" and "right =>" 29 JPanel jpButtons = new JPanel(); 30 jpButtons.setLayout( new FlowLayout()); 31 jpButtons.add(jbtLeft); 32 jpButtons.add(jbtRight); 33 34 // Set keyboard mnemonics 35 jbtLeft.setMnemonic( 'L' ); 36 jbtRight.setMnemonic( 'R' ); 37 38 // Set icons and remove text 39 // jbtLeft.setIcon(new ImageIcon("image/left.gif")); 40 // jbtRight.setIcon(new ImageIcon("image/right.gif")); 41 // jbtLeft.setText(null); 42 // jbtRight.setText(null); 43 44 // Set tool tip text on the buttons 45 jbtLeft.setToolTipText( "Move message to left" ); 46 jbtRight.setToolTipText( "Move message to right" ); 47 48 // Place panels in the frame 49 setLayout( new BorderLayout()); 50 add(messagePanel, BorderLayout.CENTER); 51 add(jpButtons, BorderLayout.SOUTH); 52 53 // Register listeners with the buttons 54 jbtLeft.addActionListener( new ActionListener() { 55 public void actionPerformed(ActionEvent e) { 56 messagePanel.moveLeft(); 57 } 58 }); 59 jbtRight.addActionListener( new ActionListener() { 60 public void actionPerformed(ActionEvent e) { 61 messagePanel.moveRight(); 62 } 63 }); 64 } 65 } |
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 39 “42:
// 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 15.10(a). "image/left.gif" is located in "c:\book\image\left.gif" . Note that the back slash is the Windows file path notation. In Java, the forward slash should be used.
Figure 15.10. You can set an icon on a JButton and access a button using mnemonic keys.
You can set text and an icon on a button at the same time, if you wish, as shown in Figure 15.10(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 35). If you change the left button text to "Left" and the right button to "Right," the L and R in the captions of these buttons will be underlined , as shown in Figure 15.10(b).
Each button has a tool-tip text (lines 45 “46), which appears when the mouse is set on the button without clicking, as shown in Figure 15.10(c).
Note
| | Since MessagePanel is not in the Java API, you should place MessagePanel.java in the same directory with ButtonDemo.java. |
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