"FirebaseDatabase.getInstance().setPersistenceEnabled(true);"

"FirebaseDatabase.getInstance().setPersistenceEnabled(true);"

private static FirebaseDatabase fbDB;

    @Override
        protected void onCreate(Bundle savedInstanceState) {

            if(fbDB == null) {
                fbDB = FirebaseDatabase.getInstance();
                fbDB.setPersistenceEnabled(true);
            }
         ...
         ...
        }