Learning Java
Chapter 14: Using Swing Components
Menus
A JMenu is a standard pull-down menu with a fixed name. Menus can hold other menus as submenu items, enabling you to implement complex menu structures. In Swing, menus are first-class components, just like everything else. You can place them wherever a component would go. Another class, JMenuBar, holds menus in a horizontal bar. Menu bars are real components, too, so you can place them wherever you want in a container: top, bottom, or middle. But in the middle of a container, it usually makes more sense to use a JComboBox rather than some kind of menu.
Menu items may have associated images and shortcut keys; there are even menu items that look like checkboxes and radio buttons. Menu items are really a kind of button. Like buttons, menu items fire action events when they are selected. You can respond to menu items by registering action listeners with them.
There are two ways to use the keyboard with menus. The first is called mnemonics. A mnemonic is one character in the menu name. If you hold down the Alt key and type a menu's mnemonic, the menu will drop down, just as if you had clicked on it with the mouse. Menu items may also have mnemonics. Once a menu is dropped down, you can select individual items in the same way.
Menu items may also have accelerators. An accelerator is a key combination that selects the menu item, whether or not the menu that contains it is showing. A common example is the accelerator Ctrl-C, which is frequently used as a shortcut for the
Copy item in the
Edit menu.
The following example demonstrates several different features of menus. It creates a menu bar with three different menus. The first,
Utensils, contains several menu items, a submenu, a separator, and a
Quit item that includes both a mnemonic and an accelerator. The second menu,
Spices, contains menu items that look and act like checkboxes. Finally, the
Cheese menu demonstrates how radio button menu items can be used.
This application is shown in Figure 14-4 with one of its menus dropped down. Choosing
Quit from the menu (or pressing Ctrl-Q) removes the window. Give it a try.
//file: DinnerMenu.java
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class DinnerMenu extends JFrame {
public DinnerMenu( ) {
super("DinnerMenu v1.0");
setSize(200, 200);
setLocation(200, 200);
// create the Utensils menu
JMenu utensils = new JMenu("Utensils");
utensils.setMnemonic(KeyEvent.VK_U);
utensils.add(new JMenuItem("Fork"));
utensils.add(new JMenuItem("Knife"));
utensils.add(new JMenuItem("Spoon"));
JMenu hybrid = new JMenu("Hybrid");
hybrid.add(new JMenuItem("Spork"));
hybrid.add(new JMenuItem("Spife"));
hybrid.add(new JMenuItem("Knork"));
utensils.add(hybrid);
utensils.addSeparator( );
// do some fancy stuff with the Quit item
JMenuItem quitItem = new JMenuItem("Quit");
quitItem.setMnemonic(KeyEvent.VK_Q);
quitItem.setAccelerator(
KeyStroke.getKeyStroke(KeyEvent.VK_Q, Event.CTRL_MASK));
quitItem.addActionListener(new ActionListener( ) {
public void actionPerformed(ActionEvent e) { System.exit(0); }
});
utensils.add(quitItem);
// create the Spices menu
JMenu spices = new JMenu("Spices");
spices.setMnemonic(KeyEvent.VK_S);
spices.add(new JCheckBoxMenuItem("Thyme"));
spices.add(new JCheckBoxMenuItem("Rosemary"));
spices.add(new JCheckBoxMenuItem("Oregano", true));
spices.add(new JCheckBoxMenuItem("Fennel"));
// create the Cheese menu
JMenu cheese = new JMenu("Cheese");
cheese.setMnemonic(KeyEvent.VK_C);
ButtonGroup group = new ButtonGroup( );
JRadioButtonMenuItem rbmi;
rbmi = new JRadioButtonMenuItem("Regular", true);
group.add(rbmi);
cheese.add(rbmi);
rbmi = new JRadioButtonMenuItem("Extra");
group.add(rbmi);
cheese.add(rbmi);
rbmi = new JRadioButtonMenuItem("Blue");
group.add(rbmi);
cheese.add(rbmi);
// create a menu bar and use it in this JFrame
JMenuBar menuBar = new JMenuBar( );
menuBar.add(utensils);
menuBar.add(spices);
menuBar.add(cheese);
setJMenuBar(menuBar);
}
public static void main(String[] args) {
JFrame f = new DinnerMenu( );
f.addWindowListener(new WindowAdapter( ) {
public void windowClosing(WindowEvent we) { System.exit(0); }
});
f.setVisible(true);
}
}
Figure 14-4.
The DinnerMenu application
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Yes, we know.
Quit doesn't belong in the
Utensils menu. If it's driving you crazy, you can go back and add a
File menu as an exercise when we're through.
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