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A week after the author was notified in the comments that his diagram is inaccurate, it still has not been corrected. Therefore I vote this down.

The Diagram is accurate. you can look at the Adobe Documentation.

http://livedocs.adobe.com/flex/201/html/wwhelp/wwhimpl/common/html/wwhelp.htm?context=LiveDocs_Book_Parts&file=components_056_03.html

Look at the source code for UIComponent, in Flex 2.0.1:

public class UIComponent extends FlexSprite

The Adobe diagram is wrong, or misleading at best, since one might argue that the accompanying text "The following image shows this hierarchy in detail up to the Flash Sprite component level" implies that there might be "detail" left out beyond the Sprite level. Your post makes no such qualification, but instead explicitly says "UIComponent extends Sprite" which is just plain not true, unless you want to argue that having Sprite as an ancestor is the same as extending it. (In which case yoou could say "UIComponent extends Object" and still be accurate.)

Updated Thanks