java.util.Optional (Java8부터 도입) Optional is primarily intended for use as a method return type where there is a clear need to represent "no result" and where using null is likely to cause errors. A variable whose type is Optional should never itself be null. it should always point to an Optional instance. (Java SE 9 & JDK 9 / Optional API Note 참조) 옵셔널은 주로 "결과 없음"을 나타낼 필요가 분명하고 null을 사용하면 오류가 발생할..