effective java
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Item 8: hashCode 재정의
equals(null)일 때 true가 발생할 가능성은 없음.
Item 38: Adhere to generally accepted naming conventions
Note that constant fields constitute the only recommended use of underscores. (p.125)
Naming convention : http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.8