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Chapter 10 Would You Like to Choose from the Menu? |
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Chapter 10 Would You Like to Choose from the Menu? |
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Now that you know about all the different menu classes, it is time to show an example. Example 10.1 contains the code to put up a functional MenuBar attached to a Frame, using the 1.0 event model. Figure 10.2 (earlier in the chapter) displays the resulting screen. The key parts to examine are how the menus are put together in the MenuTest constructor and how their actions are handled within action(). I implement one real action in the example: the one that terminates the application when the user chooses Quit. Any other action just displays the label of the item and (if it was a CheckBoxMenuItem) the item's state, to give you an idea of how you can use the information returned in the event.
import java.awt.*;
public class MenuTest extends Frame {
MenuTest () {
super ("MenuTest");
MenuItem mi;
Menu file = new Menu ("File", true);
file.add ("Open");
file.add (mi = new MenuItem ("Close"));
mi.disable();
Menu extras = new Menu ("Extras", false);
extras.add (new CheckboxMenuItem ("What"));
extras.add ("Yo");
extras.add ("Yo");
file.add (extras);
file.addSeparator();
file.add ("Quit");
Menu help = new Menu("Help");
help.add ("About");
MenuBar mb = new MenuBar();
mb.add (file);
mb.add (help);
mb.setHelpMenu (help);
setMenuBar (mb);
resize (200, 200);
}
public boolean handleEvent (Event e) {
if (e.id == Event.WINDOW_DESTROY) {
System.exit(0);
}
return super.handleEvent (e);
}
public boolean action (Event e, Object o) {
if (e.target instanceof MenuItem) {
if ("Quit".equals (o)) {
dispose();
System.exit(1);
} else {
System.out.println ("User selected " + o);
if (e.target instanceof CheckboxMenuItem) {
CheckboxMenuItem cb = (CheckboxMenuItem)e.target;
System.out.println ("The value is: " + cb.getState());
}
}
return true;
}
return false;
}
public static void main (String []args) {
MenuTest f = new MenuTest ();
f.show();
}
}
The MenuTest constructor builds all the menus, creates a menu bar, adds the menus to the menu bar, and adds the menu bar to the Frame. To show what is possible, I've included a submenu, a separator bar, a disabled item, and a help menu.
The handleEvent() method exists to take care of WINDOW_DESTROY events, which are generated if the user uses a native command to exit from the window.
The action() method does the work; it received the action events generated whenever the user selects a menu. We ignore most of them, but a real application would need to do more work figuring out the user's selection. As it is, action() is fairly simple. If the user selected a menu item, we check to see whether the item's label was "Quit"; if it was, we exit. If the user selected anything else, we print the selection and return true to indicate that we handled the event.
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.
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PopupMenu |
