item23

Item 23: Prefer Class Hierarchies to Tagged Classes

Tagged class is a class whose instances come in multiple flavors and contain a tag field indicating the flavor of the instance.
The data and operations, i.e. the type, of a tagged class is dependent on the tag instead of dynamic subtyping; this results in many shortcomings:
- As opposed to defining a new subclass, adding a new flavor is an extreme nuisance, because the tagged class has to be directly modified to make every switch statement handle another flavor.
- Implementation details of each flavor are jumbled together into a single class, not only cluttering the source code, but also increasing memory footprint with unused members.
Replace them with class hierarchies and subtyping as we know them.