This lecture is not a simple test lecture. I was learning and applying how to use JUnit, Mockito, etc. to write tests. Then, I became curious about good test code and well-written test code beyond simply using test tools, and I found this lecture. I had enjoyed watching the instructor's first lecture, so I had a high level of trust in it, and the contents of the table of contents seemed interesting. I took all the lectures and am leaving a review. This lecture is not a simple test lecture, but a very great lecture that helps me think about good architecture and OOP. For me, whose purpose of writing tests was simply to prevent regression, testing and design are mutually complementary, and it made me think about good design naturally while informing me of the problems of the existing layered architecture, and the limitations of tests written in the layered architecture. It does not just point out problems in theory, but suggests better structures and test code writing through the process of refactoring code. I was learning Mockito for the first time recently and was impressed by the tests using stubbing. This was a great experience for me to test the tests that required Mockito in pure Java without an external library. In the end, it is a good lecture that continues the words of the first part that everything is OOP. In addition, it was good to talk about the concepts needed for testing and the opinions that contained the instructor's personal concerns. I learned a lot because I could indirectly feel the seriousness and attitude toward the development field. Thank you for the great lecture.