31.17. Programming Exercises

[Page 968 ( continued )]

29.2. Menus

Menus make selection easier and are widely used in window applications. Java provides five classes that implement menus: JMenuBar , JMenu , JMenuItem , JCheckBoxMenuItem , and JRadioButtonMenuItem .

JMenuBar is a top-level menu component used to hold menus. A menu consists of menu items that the user can select (or toggle on or off). A menu item can be an instance of JMenuItem , JCheckBoxMenuItem , or JRadioButtonMenuItem . Menu items can be associated with icons, keyboard mnemonics , and keyboard accelerators. Menu items can be separated using separators.

29.2.1. Creating Menus

The sequence of implementing menus in Java is as follows :

Create a menu bar and associate it with a frame or an applet by using the setJMenuBar method. For example, the following code creates a frame and a menu bar, and sets the menu bar in the frame:

 JFrame frame =   new   JFrame(); frame.setSize(   300   ,   200   ); frame.setVisible(   true   ); JMenuBar jmb =   new   JMenuBar(); frame.setJMenuBar(jmb);  // Attach a menu bar to a frame

Create menus and associate them with the menu bar. You can use the following constructor to create a menu:
```
   public   JMenu(String label) 
```
Here is an example of creating menus:
```
 JMenu fileMenu =   new   JMenu(   "File"   ); JMenu helpMenu =   new   JMenu(   "Help"   ); 
```
This creates two menus labeled File and Help , as shown in Figure 29.1(a). The menus will not be seen until they are added to an instance of JMenuBar , as follows:
```
 jmb.add(fileMenu); jmb.add(helpMenu); 
```
Figure 29.1. (a) The menu bar appears below the title bar on the frame. (b) Clicking a menu on the menu bar reveals the items under the menu. (c) Clicking a menu item reveals the submenu items under the menu item.
[Page 969]

Create menu items and add them to the menus.

 fileMenu.add(   new   JMenuItem(   "New"   )); fileMenu.add(   new   JMenuItem(   " Open "   )); fileMenu.addSeparator(); fileMenu.add(   new   JMenuItem(   "Print"   )); fileMenu.addSeparator(); fileMenu.add(   new   JMenuItem(   "Exit"   ));

This code adds the menu items New, Open, a separator bar, Print, another separator bar, and Exit, in this order, to the File menu, as shown in Figure 29.1(b). The addSeparator() method adds a separator bar in the menu.

3.1.

Creating submenu items. You can also embed menus inside menus so that the embedded menus become submenus. Here is an example:

 JMenu softwareHelpSubMenu =   new   JMenu(   "Software"   ); JMenu hardwareHelpSubMenu =   new   JMenu(   "Hardware"   ); helpMenu.add(softwareHelpSubMenu); helpMenu.add(hardwareHelpSubMenu); softwareHelpSubMenu.add(   new   JMenuItem(   "Unix"   )); softwareHelpSubMenu.add(   new   JMenuItem(   "NT"   )); softwareHelpSubMenu.add(   new   JMenuItem(   "Win95"   ));

This code adds two submenus, softwareHelpSubMenu and hardwareHelpSubMenu , in helpMenu . The menu items Unix , NT , and Win95 are added to softwareHelpSubMenu (see Figure 29.1(c)).

3.2.

Creating check box menu items. You can also add a JCheckBoxMenuItem to a JMenu . JCheckBoxMenuItem is a subclass of JMenuItem that adds a Boolean state to the JMenuItem , and displays a check when its state is true. You can click a menu item to turn it on or off. For example, the following statement adds the check box menu item Check it (see Figure 29.2(a)):

 helpMenu.add(   new   JCheckBoxMenuItem(   "Check it"   ));

Figure 29.2. (a) A check box menu item lets you check or uncheck a menu item just like a check box. (b) You can use JRadioButtonMenuItem to choose among mutually exclusive menu choices. (c) You can set image icons, keyboard mnemonics, and keyboard accelerators in menus.

[Page 970]

3.3.

Creating radio button menu items. You can also add radio buttons to a menu, using the JRadioButtonMenuItem class. This is often useful when you have a group of mutually exclusive choices in the menu. For example, the following statements add a submenu named Color and a set of radio buttons for choosing a color (see Figure 29.2(b)):

 JMenu colorHelpSubMenu =   new   JMenu(   "Color"   ); helpMenu.add(colorHelpSubMenu); JRadioButtonMenuItem jrbmiBlue, jrbmiYellow, jrbmiRed; colorHelpSubMenu.add(jrbmiBlue =   new   JRadioButtonMenuItem(   "Blue"   )); colorHelpSubMenu.add(jrbmiYellow =   new   JRadioButtonMenuItem(   "Yellow"   )); colorHelpSubMenu.add(jrbmiRed =   new   JRadioButtonMenuItem(   "Red"   )); ButtonGroup btg =   new   ButtonGroup(); btg.add(jrbmiBlue); btg.add(jrbmiYellow); btg.add(jrbmiRed);

The menu items generate ActionEvent . Your listener class must implement the ActionListener and the actionPerformed handler to respond to the menu selection.

29.2.2. Image Icons, Keyboard Mnemonics, and Keyboard Accelerators

The menu components JMenu , JMenuItem , JCheckBoxMenuItem , and JRadioButtonMenuItem have the icon and mnemonic properties. For example, using the following code, you can set icons for the New and Open menu items, and set keyboard mnemonics for File, Help, New, and Open:

 JMenuItem jmiNew, jmiOpen; fileMenu.add(jmiNew =   new   JMenuItem(   "New"   )); fileMenu.add(jmiOpen =   new   JMenuItem(   "Open"   )); jmiNew.setIcon(   new   ImageIcon(   "image/new.gif"   )); jmiOpen.setIcon(   new   ImageIcon(   "image/open.gif"   )); helpMenu.setMnemonic(   'H'   ); fileMenu.setMnemonic(   'F'   ); jmiNew.setMnemonic(   'N'   ); jmiOpen.setMnemonic(   'O'   );

The new icons and mnemonics are shown in Figure 29.2(c). You can also use JMenuItem constructors like the ones that follow to construct and set an icon or mnemonic in one statement:

   public   JMenuItem(String label, Icon icon);   public   JMenuItem(String label,   int   mnemonic);

By default, the text is at the right of the icon. Use setHorizontalTextPosition(SwingConstants.LEFT) to set the text to the left of the icon.

To select a menu, press the ALT key and the mnemonic key. For example, press ALT+F to select the File menu, and then press ALT+O to select the Open menu item. Keyboard mnemonics are useful, but only let you select menu items from the currently open menu. Key accelerators, however, let you select a menu item directly by pressing the CTRL and accelerator keys. For example, by using the following code, you can attach the key CTRL+O to the Open menu item:

 jmiOpen.setAccelerator(KeyStroke.getKeyStroke (KeyEvent.VK_O, ActionEvent.CTRL_MASK));

The setAccelerator method takes a KeyStroke object. The static method getKeyStroke in the KeyStroke class creates an instance of the keystroke. VK_O is a constant representing the O key, and CTRL_MASK is a constant indicating that the CTRL key is associated with the keystroke.

[Page 971]

Note

As shown in Figure 15.3, AbstractButton is the superclass for JButton and JMenuItem , and JMenuItem is a superclass for JCheckBoxMenuItem , JMenu , and JRadioButtonMenuItem . The menu components are very similar to buttons.

29.2.3. Example: Using Menus

This section gives an example that creates a user interface to perform arithmetic. The interface contains labels and text fields for Number 1, Number 2, and Result. The Result text field displays the result of the arithmetic operation between Number 1 and Number 2. Figure 29.3 contains a sample run of the program.

Figure 29.3. Arithmetic operations can be performed by clicking buttons or by choosing menu items from the Operation menu.

Here are the major steps in the program (Listing 29.1):

1.	Create a menu bar and set it in the applet. Create the menus Operation and Exit, and add them to the menu bar. Add the menu items Add, Subtract, Multiply, and Divide under the Operation menu, and add the menu item Close under the Exit menu.
2.	Create a panel to hold labels and text fields, and place the panel in the center of the applet.
3.	Create a panel to hold the four buttons labeled Add, Subtract, Multiply, and Divide. Place the panel in the south of the applet.
4.	Implement the actionPerformed handler to process the events from the menu items and the buttons.

Listing 29.1. MenuDemo.java

(This item is displayed on pages 971 - 974 in the print version)

 1   import   java.awt.*; 2   import   java.awt.event.*; 3   import   javax.swing.*; 4 5   public class   MenuDemo   extends   JApplet { 6  // Text fields for Number 1, Number 2, and Result  7   private   JTextField jtfNum1, jtfNum2, jtfResult; 8 9  // Buttons "Add", "Subtract", "Multiply" and "Divide"  10   private   JButton jbtAdd, jbtSub, jbtMul, jbtDiv; 11 12  // Menu items "Add", "Subtract", "Multiply", "Divide" and "Close"  13    private   JMenuItem jmiAdd, jmiSub, jmiMul, jmiDiv, jmiClose;  14 15   public   MenuDemo() { 16  // Create menu bar  17  JMenuBar jmb =   new   JMenuBar();

[Page 972]

 18 19  // Set menu bar to the applet  20  setJMenuBar(jmb);  21 22  // Add menu "Operation" to menu bar  23  JMenu operationMenu =   new   JMenu(   "Operation"   );  24  operationMenu.setMnemonic(   'O'   );  25  jmb.add(operationMenu);  26 27  // Add menu "Exit" to menu bar  28  JMenu exitMenu =   new   JMenu(   "Exit"   );  29  exitMenu.setMnemonic(   'E'   );  30  jmb.add(exitMenu);  31 32  // Add menu items with mnemonics to menu "Operation"  33  operationMenu.add(jmiAdd=   new   JMenuItem(   "Add"   ,   'A'   ));  34 operationMenu.add(jmiSub =   new   JMenuItem(   "Subtract"   ,   'S'   )); 35 operationMenu.add(jmiMul =   new   JMenuItem(   "Multiply"   ,   'M'   )); 36 operationMenu.add(jmiDiv =   new   JMenuItem(   "Divide"   ,   'D'   )); 37 exitMenu.add(jmiClose =   new   JMenuItem(   "Close"   ,   'C'   )); 38 39  // Set keyboard accelerators  40  jmiAdd.setAccelerator(  41  KeyStroke.getKeyStroke(KeyEvent.VK_A, ActionEvent.CTRL_MASK));  42 jmiSub.setAccelerator( 43 KeyStroke.getKeyStroke(KeyEvent.VK_S, ActionEvent.CTRL_MASK)); 44 jmiMul.setAccelerator( 45 KeyStroke.getKeyStroke(KeyEvent.VK_M, ActionEvent.CTRL_MASK)); 46 jmiDiv.setAccelerator( 47 KeyStroke.getKeyStroke(KeyEvent.VK_D, ActionEvent.CTRL_MASK)); 48 49  // Panel p1 to hold text fields and labels  50 JPanel p1 =   new   JPanel(   new   FlowLayout()); 51 p1.add(   new   JLabel(   "Number 1"   )); 52 p1.add(jtfNum1 =   new   JTextField(   3   )); 53 p1.add(   new   JLabel(   "Number 2"   )); 54 p1.add(jtfNum2 =   new   JTextField(   3   )); 55 p1.add(   new   JLabel(   "Result"   )); 56 p1.add(jtfResult =   new   JTextField(   4   )); 57 jtfResult.setEditable(   false   ); 58 59  // Panel p2 to hold buttons  60 JPanel p2 =   new   JPanel(   new   FlowLayout()); 61 p2.add(jbtAdd =   new   JButton(   "Add"   )); 62 p2.add(jbtSub =   new   JButton(   "Subtract"   )); 63 p2.add(jbtMul =   new   JButton(   "Multiply"   )); 64 p2.add(jbtDiv =   new   JButton(   "Divide"   )); 65 66  // Add panels to the frame  67 setLayout(   new   BorderLayout()); 68 add(p1, BorderLayout.CENTER); 69 add(p2, BorderLayout.SOUTH); 70 71  // Register listeners  72  jbtAdd.addActionListener(   new   ActionListener() {  73   public void   actionPerformed(ActionEvent e) { 74 calculate(   '+'   ); 75 } 76 });

[Page 973]

 77  jbtSub.addActionListener(   new   ActionListener() {  78   public void   actionPerformed(ActionEvent e) { 79 calculate(   '-'   ); 80 } 81 }); 82  jbtMul.addActionListener(   new   ActionListener() {  83   public void   actionPerformed(ActionEvent e) { 84 calculate(   '*'   ); 85 } 86 }); 87  jbtDiv.addActionListener(   new   ActionListener() {  88   public void   actionPerformed(ActionEvent e) { 89 calculate(   '/'   ); 90 } 91 }); 92  jmiAdd.addActionListener(   new   ActionListener() {  93   public void   actionPerformed(ActionEvent e) { 94 calculate(   '+'   ); 95 } 96 }); 97  jmiSub.addActionListener(   new   ActionListener() {  98   public void   actionPerformed(ActionEvent e) { 99 calculate(   '-'   ); 100 } 101 }); 102  jmiMul.addActionListener(   new   ActionListener() {  103   public void   actionPerformed(ActionEvent e) { 104 calculate(   '*'   ); 105 } 106 }); 107  jmiDiv.addActionListener(   new   ActionListener() {  108   public void   actionPerformed(ActionEvent e) { 109 calculate(   '/'   ); 110 } 111 }); 112  jmiClose.addActionListener(   new   ActionListener() {  113   public void   actionPerformed(ActionEvent e) { 114 System.exit(     ); 115 } 116 }); 117 } 118 119  /** Calculate and show the result in jtfResult */  120   private void   calculate(   char   operator) { 121  // Obtain Number 1 and Number 2  122   int   num1 = (Integer.parseInt(jtfNum1.getText().trim())); 123   int   num2 = (Integer.parseInt(jtfNum2.getText().trim())); 124   int   result =     ; 125 126  // Perform selected operation  127   switch   (operator) { 128   case     '+'   : result = num1 + num2; 129   break   ; 130   case     '-'   : result = num1 - num2; 131   break   ; 132   case     '*'   : result = num1 * num2; 133   break   ; 134   case     '/'   : result = num1 / num2; 135 }

[Page 974]

 136 137  // Set result in jtfResult  138 jtfResult.setText(String.valueOf(result)); 139 } 140 }

The program creates a menu bar, jmb , which holds two menus: operationMenu and exitMenu (lines 17 “30). The operationMenu contains four menu items for doing arithmetic: Add, Subtract, Multiply, and Divide. The exitMenu contains the menu item Close for exiting the program. The menu items in the Operation menu are created with keyboard mnemonics and accelerators.

The user enters two numbers in the number fields. When an operation is chosen from the menu, its result, involving two numbers, is displayed in the Result field. The user can also click the buttons to perform the same operation.

The private method calculate(char operator) (lines 120 “139) retrieves operands from the text fields in Number 1 and Number 2, applies the binary operator on the operands, and sets the result in the Result text field.