Container.getComponentZOrder() should be non-final. There are certain situations where applications insert components in between a Container and the Component being added as a child. It is helpful to the end user to hide this fact as much as possible, although it is not possible to hide it completely. One place where it can not be hidden is Container.getComponentZOrder(), which is non-final. A subclass might want to, for example, take the passed Component, fetch its parent, and then call super.getComponentZOrder() passing the parent.
I've added an attachment of the source code which would use the API in question. The intent is to provide a JDesktopPane replacement which renders all of the children components to a privately-managed VolatileImage offscreen buffer so that their contents can be composited as a post-process. Ideally this JDesktopPane replacement would simply be able to render arbitrary Swing components as children. However, due to the need to return a different Graphics object on behalf of the child components, and because Component.getGraphics() gives no indication of whether it was called on behalf of a child component, it is necessary to interpose on this call by inserting a component in between the desktop pane and the actual child component. It is the presence of this intermediate component which we desire to hide as much as possible from the end user by changing most methods which accept a Component as argument to try to walk up the hierarchy one level before calling the superclass's method. getComponentZOrder is a key method where this kind of interposition is not possible.