It was very helpful for my studies. However, the lecture content feels a little unfriendly. However, if you follow the lecture diligently, you will be able to implement a chat window. (As of September 21) If you search for Android Extension and install it in Android Studio before taking the class, you will be able to follow the class more comfortably. // This is a lecture that can be helpful for those who want to study about implementing a chat window.