I only heard about ReactiveX, which is reactive programming, and I bought it on impulse this time. Usually, I am rarely satisfied with impulse purchases, but this lecture was very interesting. I am not good at object-oriented programming, so the concept was a bit difficult for me, but I worked hard with the goal of trying one last fun example. I hope to find a project that can use reactive programming soon. Since I have a clearer understanding of the advantages of functional programming than before, I think I will use functional programming more actively than before. It was a very interesting and helpful lecture for me.