addHierarchyListenermethod. When the hierarchy to which the Component belongs changes, the
hierarchyChangedmethod in the listener object is invoked, and the
HierarchyEventis passed to it.
Hierarchy events are provided for notification purposes ONLY;
The AWT will automatically handle changes to the hierarchy internally so
that GUI layout, displayability, and visibility work properly regardless
of whether a program registers a
HierarchyListener or not.