2021
https://www.baeldung.com/java-optional
    https://habr.com/ru/company/otus/blog/668794/
2020
https://levelup.gitconnected.com/java-optional-is-not-so-obvious-263d9559dd41
    https://habr.com/ru/post/540080/
2019
https://dzone.com/articles/java-optional