FS DB Reading String

        //Getting instance
        FirebaseFirestore db = FirebaseFirestore.getInstance();

        //Getting Reference to "new doc" document
        //You can get the path in this way too, lot easier!
        DocumentReference documentReference = db.collection("users").document("new doc");

        //Calling the get() method with a callback function
        documentReference.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
            public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                //Checking request result
                if (task.isSuccessful()) {
                    //Request was successful but it never means that data is found
                    DocumentSnapshot data = task.getResult();
                    if (data.exists()) {
                        //document named "new doc" found

                        //We know in which format we have writen data to that document, so we will get it in same manner
                        //We used map to store that value, now we will get that
                        Map<String, Object> map = data.getData();

                        //We know the value "waqas" was stored against key "username"
                        String username = (String) map.get("username");

                        Toast.makeText(MainActivity.this, "username: " + username, Toast.LENGTH_SHORT).show();

                    } else {
                        //document with name "new doc" not found at the path specified.

                } else {
                    //Request was not successful
                    //Could be some rules or internet problem
                    Log.i(TAG, "onComplete: Request unsuccessful, error: " + task.getException().getLocalizedMessage());