4주차 참고 자료입니다. 

# 참고자료

## singleton의 사용
https://stackoverflow.com/questions/24351046/java-singleton-usage

## static vs static final 
https://medium.com/@chuck.m.park/final-vs-static-final-in-java-cf73e6cc6848

## wrapper 클래스
https://hyeonstorage.tistory.com/168