Create a Main Menu - Swing

Create a Main Menu

Traditionally, the most commonly used menu is the main menu. This is the menu defined by the menu bar, and it is the menu that defines all (or nearly all) of the functionality of an application. Fortunately, Swing makes creating and managing the main menu easy. This section shows you how to construct a basic main menu. Subsequent sections will show you how to add options to it.

Constructing the main menu requires several steps. First, create the JMenuBar object that will hold the menus. Next, construct each menu that will be in the menu bar. In general, a menu is constructed by first creating a JMenu object and then adding JMenuItems to it. After the menus have been created, add them to the menu bar. The menu bar, itself, must then be added to the frame by calling setJMenuBar( ). Finally, for each menu item, you must add an action listener that handles the action event fired when the menu item is selected.

A good way to understand the process of creating and managing menus is to work through an example. Here is a program that creates a simple menu bar that contains three menus. The first is a standard File menu that contains Open, Close, Save, and Exit selections. The second menu is called Options, and it contains two submenus called Colors and Priority. The third menu is called Help, and it has one item: About. When a menu item is selected, the name of the selection is displayed in a label in the content pane. Sample output is shown in Figure 33-1.

// Demonstrate a simple main menu.

import java.awt.*; import java.awt.event.*; import javax.swing.*;

class MenuDemo implements ActionListener {

JLabel jlab;

MenuDemo() {

// Create a new JFrame container.

JFrame jfrm = new JFrame("Menu Demo");

    //Specify FlowLayout for the layout manager.

    jfrm.setLayout(new FlowLayout());

    //Give the frame an initial size.

    jfrm.setSize(220, 200);

    //Terminate the program when the user closes the application.

    jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

     //Create a label that will display the menu selection.

jlab = new JLabel();

    //Create the menu bar.

    JMenuBar jmb = new JMenuBar();

     //Create the File menu.

JMenu jmFile = new JMenu("File"); JMenuItem jmiOpen = new JMenuItem("Open"); JMenuItem jmiClose = new JMenuItem("Close"); JMenuItem jmiSave = new JMenuItem("Save"); JMenuItem jmiExit = new JMenuItem("Exit"); jmFile.add(jmiOpen);

jmFile.add(jmiClose);

jmFile.add(jmiSave);

jmFile.addSeparator();

jmFile.add(jmiExit);

jmb.add(jmFile);

// Create the Options menu.

JMenu jmOptions = new JMenu("Options");

// Create the Colors submenu.

JMenu jmColors = new JMenu("Colors"); JMenuItem jmiRed = new JMenuItem("Red"); JMenuItem jmiGreen = new JMenuItem("Green"); JMenuItem jmiBlue = new JMenuItem("Blue"); jmColors.add(jmiRed); jmColors.add(jmiGreen); jmColors.add(jmiBlue); jmOptions.add(jmColors);

// Create the Priority submenu.

JMenu jmPriority = new JMenu("Priority"); JMenuItem jmiHigh = new JMenuItem("High"); JMenuItem jmiLow = new JMenuItem("Low"); jmPriority.add(jmiHigh); jmPriority.add(jmiLow); jmOptions.add(jmPriority);

// Create the Reset menu item.

JMenuItem jmiReset = new JMenuItem("Reset"); jmOptions.addSeparator(); jmOptions.add(jmiReset);

     //Finally, add the entire options menu to

     //the menu bar

jmb.add(jmOptions);

// Create the Help menu.

JMenu jmHelp = new JMenu("Help");

JMenuItem jmiAbout = new JMenuItem("About"); jmHelp.add(jmiAbout);

jmb.add(jmHelp);

     Add action listeners for the menu items. jmiOpen.addActionListener(this); jmiClose.addActionListener(this); jmiSave.addActionListener(this); jmiExit.addActionListener(this); jmiRed.addActionListener(this); jmiGreen.addActionListener(this); jmiBlue.addActionListener(this); jmiHigh.addActionListener(this); jmiLow.addActionListener(this); jmiReset.addActionListener(this); jmiAbout.addActionListener(this);

     Add the label to the content pane. jfrm.add(jlab);

     Add the menu bar to the frame. jfrm.setJMenuBar(jmb);

     Display the frame. jfrm.setVisible(true);

}

// Handle menu item action events.

public void actionPerformed(ActionEvent ae) {

     Get the action command from the menu selection. String comStr = ae.getActionCommand();

     If user chooses Exit, then exit the program. if(comStr.equals("Exit")) System.exit(0);

     Otherwise, display the selection. jlab.setText(comStr + " Selected");

}

public static void main(String args[]) {

// Create the frame on the event dispatching thread.

SwingUtilities.invokeLater(new Runnable() {

public void run() { new MenuDemo();

}

});

}

}

Let’s examine, in detail, how the menus in this program are created, beginning with the MenuDemo constructor. It starts by creating a JFrame and setting its layout manager, size, and default close operation. (These operations are described in Chapter 31.) A JLabel is then constructed. It will be used to display a menu selection. Next, the menu bar is constructed and a reference to it is assigned to jmb by this statement:

// Create the menu bar.

JMenuBar jmb = new JMenuBar();

Then, the File menu jmFile and its menu entries are created by this sequence:

// Create the File menu.

JMenu jmFile = new JMenu("File");

JMenuItem jmiOpen = new JMenuItem("Open");

JMenuItem jmiClose = new JMenuItem("Close");

JMenuItem jmiSave = new JMenuItem("Save");

JMenuItem jmiExit = new JMenuItem("Exit");

The names Open, Close, Save, and Exit will be shown as selections in the menu. Next, the menu entries are added to the file menu by this sequence:

jmFile.add(jmiOpen);

jmFile.add(jmiClose);

jmFile.add(jmiSave);

jmFile.addSeparator();

jmFile.add(jmiExit);

Finally, the File menu is added to the menu bar with this line:

jmb.add(jmFile);

Once the preceding code sequence completes, the menu bar will contain one entry: File. The File menu will contain four selections in this order: Open, Close, Save, and Exit. However, notice that a separator has been added before Exit. This visually separates Exit from the preceding three selections.

The Options menu is constructed using the same basic process as the File menu. However, the Options menu consists of two submenus, Colors and Priority, and a Reset entry. The submenus are first constructed individually and then added to the Options menu. The Reset item is added last. Then, the Options menu is added to the menu bar. The Help menu is constructed using the same process.

Notice that MenuDemo implements the ActionListener interface and action events generated by a menu selection are handled by the actionPerformed( ) method defined by MenuDemo. Therefore, the program adds this as the action listener for the menu items. Notice that no listeners are added to the Colors or Priority items because they are not actually selections. They simply activate submenus.

Finally, the menu bar is added to the frame by the following line:

jfrm.setJMenuBar(jmb);

As mentioned, menu bars are not added to the content pane. They are added directly to the JFrame.

The actionPerformed( ) method handles the action events generated by the menu. It obtains the action command string associated with the selection by calling getActionCommand( ) on the event. It stores a reference to this string in comStr. Then, it tests the action command against "Exit", as shown here:

if(comStr.equals("Exit")) System.exit(0);

If the action command is "Exit", then the program terminates by calling System.exit( ). This method causes the immediate termination of a program and passes its argument as a status code to the calling process, which is usually the operating system or the browser. By convention, a status code of zero means normal termination. Anything else indicates that the program terminated abnormally. For all other menu selections, the choice is displayed.

At this point, you might want to experiment a bit with the MenuDemo program. Try adding another menu or adding additional items to an existing menu. It is important that you understand the basic menu concepts before moving on because this program will evolve throughout the course of this chapter.