See MartinFowler'''''''s site at http://refactoring.com/catalog/replaceTypeCodeWithClass.html
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One potential problem with the blood-type example is that the "types" may be managed and entered by non-programmers. If every time a "type" needs to be created or changed, and a call to a programmer must go out, it could slow up productivity and cost the org. Blood types do not change that often, but each domain category is different. Be careful before making every category into class set: have a ''reason'' to do it first, not just because "Mr. Fowler said so". Related: CompilingVersusMetaDataAid --Top

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CategoryConditionalsAndDispatching