Chapter1 - Enum pattern

Fixed 된 String을 Enum Class로 정의 해서 사용하자. 

An enum pattern, in which each enumerated case is associated to an instance of a generic attribute class, is identified and implemented. It is particularly useful in situations where there is a finite number of cases and each case is characterized by a set of constants.

http://www.codeproject.com/Articles/38666/Enum-Pattern