Design Patterns Explained

52 Part II • The Limitations of Traditional Object-Oriented Design
Polymorphism isn 't
needed at all levels
High-level class
diagram
A more detailed look
at the CAD/CAM
systems
Figure 3-5 High-level view of my solution.
In other words, the application can initalize everything so that the
expert system uses the appropriate CAD/CAM system. However,
the expert system has to be able to use either version. Hence, I need
to make V1 and V2 look the same to the expert system.
Although polymorphism is definitely needed at the geometry
extractor level, it will not be needed at the feature level. This is
because the expert system needs to know what type of features it is
dealing with. However, we don't want to make any changes to the
expert system when Version 3 of the CAD/CAM system comes out.
A basic understanding of object-oriented design implies that I
will have a high-level class diagram similar to the one shown in
Figure 3-6.
In other words, the expert system relates to the CAD/CAM systems
through the Model class. The Main class takes care of instantiating
the correct version of the Model (that is, V1Model or V2Model).
Now, I will describe the CAD/CAM systems and how they work.
Unfortunately , the two are very different beasts.