Create Test Cases from Classes

I have some Java classes that i need to write test cases for them(STLC not junit), i read some tutorials about it in https://www.softwaretestinghelp.com/how-to-write-effective-test-cases-test-cases-procedures-and-definitions/ but they mention there features not already implemented classes. You can see what kind of test cases i want here https://www.youtube.com/watch?v=g0PrXoWKM2Y. I am really confused right now any help Please?

I don't have features and it is just a homework so i cannot ask for more details.

I need to know how to write test cases from normal classes POJO classes(get/set)