It's a quality lecture that's hard to believe is free. In my personal opinion, there may be moments when a complete beginner takes the lecture. (Design patterns, JVM, using telnet naturally as a terminal, etc.) However, since he explains it in such detail and says that you don't need to know the deep parts of JVM right away, I think even a complete beginner can follow along if they put in the effort. I think it would be great for those who are trying to review in an ambiguous state like me. However, since the basic lecture nature does not explain the details of Generics' wildcards, streams, and threads, it would be good to look up these parts separately in a basic book or on the Internet. After completing the lecture, the following three things still linger in my head and come to mind immediately. 1. Refer to the child type as the parent type! When overriding a method, always use the child type! 2. Easy to replace when using a collection! For example, refer to the ArrayList type as the List type 3. The main character while explaining the IO and decorator patterns! It is impressive that it shows decoration! as an Oracle document I hesitated and hesitated to write the content, but the conclusion is that I am leaving this review because I am grateful for the free and great lecture. I hope many people will be helped by this lecture.