What are the drawbacks of implementing a class inside MainActivity for android?

I'm wondering when it's not a good idea to implement a class inside MainActivity. Up until this point, I've only been using one actvitiy (Main) and so I've just been creating classes inside MainActivity.

However I'm working with more activities now and I'm planning on passing an instance of a class to another activity and back from MainActivity. Should I create a new .java file (create a new class outside of MainActivity) for this class?

What are some issues I could run into if I decide to keep my class defined in my MainActivity?

Although this question can be very opinion-based, the best practice, which has been preached several times by Google engineers and Android advocates in the annual I/O, is to keep Activity classes as simple and empty as possible. Create an architecture where functionalities are delegated into different classes, treating an Activity only as an entry point. Modularizing and abstracting your code will make it much easier to understand and maintain.

If you ask specifically about the drawbacks, then is adding unnecessary complexity into an Activity. making it is less readable and less maintainable. Of course, you can nest a class or as many as you like into an Activity, but then so that you can better decide on specific coding choices, I would recommend gaining skills in architecture design patterns, as understanding the big picture of an overall architecture will also help you to make smaller coding decisions, such as when to nest a class.