Swing Menus and Toolbars - The Definitive Guide to Java Swing - page 190

Creating Custom MenuElement Components:
The MenuElement Interface
One thing all the selectable menu components have in common is that they implement the
MenuElement interface. The JSeparator doesn't implement the interface, but that's okay because
it isn't selectable. The purpose of the MenuElement interface is to allow the MenuSelectionManager
to notify the different menu elements as a user moves around a program's menu structure.
As the following interface definition shows, the MenuElement interface is made up of
five methods:
public interface MenuElement {
public Component getComponent();
public MenuElement[] getSubElements();
public void menuSelectionChanged(boolean isInclude);
public void processKeyEvent(KeyEvent event, MenuElement path[],
MenuSelectionManager mgr);
public void processMouseEvent(MouseEvent event, MenuElement path[],
MenuSelectionManager mgr);
}
The getComponent() method returns the menu's rendering component. This is usually the
menu component itself, although that isn't a requirement. The getSubElements() method
returns an array of any menu elements contained within this element. If this menu element
isn't the top of a submenu, the method should return a zero-length array of MenuElement
objects, not null .
The menuSelectionChanged() method is called whenever the menu item is placed in or
taken out of the selection path for the menu selection manager.
The two processKeyEvent() and processMouseEvent() methods are for processing a key
event or mouse event that's generated over a menu. How your menu item processes events
depends on what the component supports. For instance, unless you support accelerators, you
probably want to respond to key events only when your menu item is in the current selection
path.
■ Note If, for example, your new menu element was something like a JComboBoxMenuItem , where the
MenuElement acted like a JComboBox , the processKeyEvent() might pass along the key character to
the KeySelectionManager . See Chapter 13 for more on the KeySelectionManager .
To demonstrate the MenuElement interface, Listing 6-6 creates a new menu component
called a JToggleButtonMenuItem . This component will look and act like a JToggleButton , although
it can be on a menu. It's important to ensure that the menu goes away once the item is selected
and that the component is displayed differently when in the current selection path.