Like Windows and MacOS, OS based on GUI consist of graphical parts which contain hierarchical structure of UserInterface. For example, Windows has followed hierarchy such as menu, tool, indicator and status bar. Furthermore, the tool bar has its followed hierarchy such as icons and list boxes. These every single parts is the "object", and has comon and unique characteristics. For instance, a button is assumed its shape square, sunken when clicked, however, its results are unique such as "OK" and "Cancel". Thus, Object Oriented structure has common and unique fetures.