在Android Studio上使用Alert Dialog的正确方法是什么？

So, i am trying to develop a little application on android, and i want to get a value from an alert dialog, the user has to choose a value between 2 integers. I am following the documentation, but i don't understand all the explanation. Here a part of my code, i don't know why it can't resolve the getActivity()So, i am trying to develop a little application