如何在磁盘中存储地理围栏？ （机器人）

I'm having a problem figuring out a way to store the geofences that my app creates, in order to load them when I run the app again. The purpose is to show a list of geofences created by the user. I tried to serialize them using Java's Serialization and I also tried to use Gson to convert them into a Json string. The problem with Gson it's when I try to load them, it gives me an error message saying that Geofence is not a class. (which is true, it's an interface). I'm having a problem figuring out a way to stor