Android : Android edit system files

I've recently started working on my first application for android, and I'm a bit stuck. My application uses the CountDownTimer class, which, it seems, malfuncions - CountDownTimer.cancel() doesn't work. I do know how to fix this by modifying the CountDownTimer.java file, but I don't have the permission to do so. I'm working in Android Studio. I couldn't find an explanation online, even though it seems quite a fundamental question, isn't it? Perhaps I'm not using the right terms?

Thank you very much for your help, and I apologize if my question's a bit ignorant haha.