Swing Menus and Toolbars - The Definitive Guide to Java Swing - page 175

Note When the JPopupMenu is displayed in either a JPanel or a Panel , the outermost window relies on
the layering effect of the JRootPane to ensure that the pop-up panel is displayed at the appropriate position
in front of the other components. Chapter 8 describes the JRootPane class in more detail.

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If you need to enable the display of a Panel , you can configure it at the individual JPopupMenu
level or for your entire applet or application. At the individual pop-up level, just set the
lightWeightPopupEnabled property to false . At the system level, this is done as follows:

// From now on, all JPopupMenus will be heavyweight
JPopupMenu.setDefaultLightWeightPopupEnabled(false);

The method must be called before creating the pop-up menu. JPopupMenu objects created
before the change will have the original value (the default is true ).

Watching for Pop-Up Menu Visibility
Like the JMenu , the JPopupMenu has a special event/listener combination to watch for when the
pop-up menu is about to become visible, invisible, or canceled. The event is PopupMenuEvent ,
and the listener is PopupMenuListener . The event class simply references the source pop-up
menu of the event.

public class PopupMenuEvent extends EventObject {
public PopupMenuEvent(Object source);
}

When a JPopupMenu fires the event, any registered PopupMenuListener objects are notified
through one of its three interface methods. This lets you customize the current menu items
based on the system state or who/what the pop-up menu invoker happens to be. The
PopupMenuListener interface definition follows:

public interface PopupMenuListener extends EventListener {
public void popupMenuCanceled(PopupMenuEvent e);
public void popupMenuWillBecomeInvisible(PopupMenuEvent e);
public void popupMenuWillBecomeVisible(PopupMenuEvent e);
}

Customizing a JPopupMenu Look and Feel
Each installable Swing look and feel provides a different JPopupMenu appearance and set of
default UIResource values. Figure 6-6 shows the appearance of the JPopupMenu component for
the preinstalled set of look and feel types: Motif, Windows, and Ocean. Notice that of the
predefined look and feel classes, only Motif uses the title property of the JPopupMenu .