I'm learning more about Java through a Udemy course and most recently learned about the abstract class, interface, and inner classes. My question is, how important is it that you absolutely understand everything about each subject, or would I be able to get away with vaguely remembering different aspects of each subject? I studied each subject separately and thoroughly yet, after a week or two of not having a reason to use them, I already started forgetting things about them. Such things that I've forgotten already that I once before was able to recall were how to access a static/non-static inner class, what the difference is between an interface and an abstract class.

Basically, is it vital that I go back, thoroughly study them again as they may be essential in programming in the future as a developer? Can I get away with simply recalling the basics like their structure and whatnot?
Note: I ask this question because, of course, starting out as a beginner, I don't know much about what will be needed in the future. For added info that may help answer my question: as for the career field, I'd be going into, still undecided, but leaning towards either android development, web development, or even data engineering. Currently pursuing a Bachelor's in SE.