Using Java 1.1 Events

Example 10.2 uses the Java 1.1 event model but is otherwise very similar to Example 10.1. Take a close look at the differences and similarities. Although the code that builds the GUI is basically the same in both examples, the event handling is completely different. The helper class MyMenuItem is necessary to simplify event handling. In Java 1.1, every menu item can be an event source, so you have to register a listener for each item. Rather than calling addActionListener() explicitly for each item, we create a subclass of MenuItem that registers a listener automatically. The listener is specified in the constructor to MyMenuItem; in this example, the object that creates the menus (MenuTest12) always registers itself as the listener. An alternative would be to override processActionEvent() in MyMenuItem, but then we'd also need to write a subclass for CheckboxMenuItem.

Having said all that, the code is relatively simple. MenuTest12 implements ActionListener so it can receive action events from the menus. As I noted previously, it registers itself as the listener for every menu item when it builds the interface. The actionPerformed() method is called whenever the user selects a menu item; the logic of this method is virtually the same as it was in Example 10.1. Notice, though, that we use getActionCommand() to read the label of the menu item. (Note also that getActionCommand() doesn't necessarily return the label; you can change the command associated with the menu item by calling setActionCommand().) Similarly, we call the event's getSource() method to get the menu item that actually generated the event; we need this to figure out whether the user selected a CheckboxMenuItem (which implements ItemSelectable).

We override processWindowEvent() so that we can receive WINDOW_CLOSING events without registering a listener. Window closings occur when the user uses the native display manager to close the application. If one of these events arrives, we shut down cleanly. To make sure that we receive window events even if there are no listeners, the MenuTest12 constructor calls enableEvents(WINDOW_EVENT_MASK).

// Java 1.1 only
import java.awt.*;
import java.awt.event.*;
public class MenuTest12 extends Frame implements ActionListener {
    class MyMenuItem extends MenuItem {
        public MyMenuItem (String s, ActionListener al) {
            super (s);
            addActionListener (al);
        }
    }
    public MenuTest12 () {
        super ("MenuTest");
        MenuItem mi;
        Menu file = new Menu ("File", true);
        file.add (new MyMenuItem ("Open", this));
        mi = file.add (new MyMenuItem ("Close", this));
        mi.setEnabled (false);
        Menu extras = new Menu ("Extras", false);
        mi = extras.add (new CheckboxMenuItem ("What"));
        mi.addActionListener(this);
        mi = extras.add (new MyMenuItem ("Yo", this));
        mi.setActionCommand ("Yo1");
        mi = extras.add (new MyMenuItem ("Yo", this));
        mi.setActionCommand ("Yo2");
        file.add (extras);
        file.addSeparator();
        file.add (new MyMenuItem ("Quit", this));
        Menu help = new Menu("Help");
        help.add (new MyMenuItem ("About", this));
        MenuBar mb = new MenuBar();
        mb.add (file);
        mb.add (help);
        mb.setHelpMenu (help);
        setMenuBar (mb);
        setSize (200, 200);
        enableEvents (AWTEvent.WINDOW_EVENT_MASK);
    }
// Cannot override processActionEvent since method of MenuItem
// Would have to subclass both MenuItem and CheckboxMenuItem
    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equals("Quit")) {
            System.exit(0);
        }
        System.out.println ("User selected " + e.getActionCommand());
        if (e.getSource() instanceof ItemSelectable) {
            ItemSelectable is = (ItemSelectable)e.getSource();
            System.out.println ("The value is: " +
                (is.getSelectedObjects().length != 0)));
        }
    }
    protected void processWindowEvent(WindowEvent e) {
        if (e.getID() == WindowEvent.WINDOW_CLOSING) {
            // Notify others we are closing
            super.processWindowEvent(e);
            System.exit(0);
        } else {
            super.processWindowEvent(e);
        }
    }
    public static void main (String []args) {
        MenuTest12 f = new MenuTest12 ();
        f.show();
    }
}

I took the opportunity when writing the 1.1 code to make one additional improvement to the program. By using action commands, you can easily differentiate between the two Yo menu items. Just call setActionCommand() to assign a different command to each item. (I used "Yo1" and "Yo2".) You could also differentiate between the items by saving a reference to each menu item, calling getSource() in the event handler, and comparing the result to the saved references. However, if the ActionListener is another class, it would need access to those references. Using action commands is simpler and results in a cleaner event handler.

The intent of the setActionCommand() and getActionCommand() methods is more for internationalization support. For example, you could use setActionCommand() to associate the command Quit with a menu item, then set the item's label to the appropriate text for the user's locality.