2020-11-17-TIL

String.split() vs StringTokenizer

https://sas-study.tistory.com/102

Java Date, Time

https://madplay.github.io/post/reasons-why-javas-date-and-calendar-was-bad

https://coding-factory.tistory.com/259

https://hyeonstorage.tistory.com/204

https://docs.oracle.com/en/java/javase/15/docs/api/java.sql/java/sql/Date.html

https://stackoverflow.com/questions/12067697/convert-current-date-to-integer

How to get hour, miniute in Java

https://stackoverflow.com/questions/2654025/how-to-get-year-month-day-hours-minutes-seconds-and-milliseconds-of-the-cur

https://stackoverflow.com/questions/8150155/java-gethours-getminutes-and-getseconds/8150681

Calendar

https://stackoverflow.com/questions/26962388/printing-a-calendar

How to get current time usinig Calendar in Java

https://stackoverflow.com/questions/6055112/why-can-i-have-only-one-instance-of-calendar-object

Calendar cal = Calendar.getInstance();
Calendar cal2 = Calendar.getInstance();
System.out.println(cal == cal2);

it’s not singleton

https://stackoverflow.com/questions/30842413/does-getinstance-denote-a-singleton-according-the-singleton-pattern/30842570

https://stackabuse.com/how-to-get-current-date-and-time-in-java/

Date refresh without create new instance

https://stackoverflow.com/questions/21859981/java-date-that-refreshes/21860245

How to clear screen in Java

https://lottoking.tistory.com/2387

https://www.quora.com/How-do-I-clear-console-screen-CMD-screen-in-Java-Is-there-any-function-in-Java-like-clrscr-and-system-cls-in-C

How to initialize/fill 2d array with 0 in Java

https://stackoverflow.com/questions/7118178/arrays-fill-with-multidimensional-array-in-java

Java set color in console

https://stackoverflow.com/questions/5762491/how-to-print-color-in-console-using-system-out-println

Map

K,V 구조의 자료구조

Map<String, Long> map = new HashMap<>();

제너릭 타입에는 객체로 전달을 해야하므로 프리미티브 타입을 객체로 사용하려면 Wrapper클래스를 사용하면 된다.

HashMap vs HashTable

https://odol87.tistory.com/3

Wrapper Class

Long x = 100L;

변수 = 리터럴 상수

자바 최신버전에서는 자동으로 오토박싱/언박식으로 자동 캐스팅을 해준다.

https://m.blog.naver.com/PostView.nhn?blogId=clown7942&logNo=110120983491&proxyReferer=https:%2F%2Fwww.google.com%2F

Java Optional

http://homoefficio.github.io/2019/10/03/Java-Optional-%EB%B0%94%EB%A5%B4%EA%B2%8C-%EC%93%B0%EA%B8%B0/

null 체크는 if문으로 체크하는 것보다 Optional을 이용하는것이 더 최신 feature

Kotlin은 Optional이 기본형이다 -> NullPointException이 예방된다.

객체를 멤버값이아닌 주소값으로 정렬을 한다던가 가끔 사용하는 경우가 있다

자바에서는 var보다 명확하게 타입을 적어주는 것이 좋다?

Process and Thread

프로그램은 항상 파일이 있고, 사용자가 어떤 작업으로 하고 다시 파일로 저장

CPU - Memory - SSD/HDD

https://magi82.github.io/process-thread/

하이퍼 스레딩

https://www.intel.co.kr/content/www/kr/ko/gaming/resources/hyper-threading.html

가상화

https://kim-dragon.tistory.com/5

Session, Connection, Transaction

https://db.apache.org/derby/docs/10.12/devguide/cdevconcepts19173.html

https://zhangyuhui.blog/2018/01/29/jpa-transaction-hibernate-session-jdbc-connection-and-db-transaction/