Firebase apps work offline and enabling disk persistence allows your app to keep all of its state even after an app restart. Firebase also provides several tools for monitoring presence and connectivity state, as we will cover in the lines bellow.
First of all, in order to be able to use Firebase on Android we need to:
Firebase is a powerful API to store and sync data in realtime helping front-end developers build realtime apps for the web. You can save data, update data, and listen for data changes in realtime with only a few lines of code. Data is stored as standard JSON and is accessible from any platform. Firebase also has safety and security built in to keep your data and your users data safe.