Application instance manages the windows that a program uses, and allows the program to switch between the active windows, as required. The
Application contains a reference to one or more
Window instances. At any given instant, one of the windows is at the foreground of the display. A
Window instance contains a reference to a single child display element. You could just display this one element if you wanted to (for example, the display could be a single string of text). However, you will frequently want to have multiple items in different areas of the display. In this case, the container will hold a number of child items, each of which will be a particular display element.
Figure 7-14 shows how everything fits ...