Generic 타입 사용법
https://namjackson.tistory.com/18
String 파일 저장
https://baejangho.com/entry/Java-save-String-to-file
Singleton
https://medium.com/@joongwon/multi-thread-%ED%99%98%EA%B2%BD%EC%97%90%EC%84%9C%EC%9D%98-%EC%98%AC%EB%B0%94%EB%A5%B8-singleton-578d9511fd42