Class Method and Static Method

http://schoolofweb.net/blog/posts/%ED%8C%8C%EC%9D%B4%EC%8D%AC-oop-part-4-%ED%81%B4%EB%9E%98%EC%8A%A4-%EB%A9%94%EC%86%8C%EB%93%9C%EC%99%80-%EC%8A%A4%ED%83%9C%ED%8B%B1-%EB%A9%94%EC%86%8C%EB%93%9C-class-method-and-static-method/