How to get Key from firebase database in android

I am working on an application where I need to get the keys from the real-time database's collection. I have stored users id's in keys inside the collection. I am performing a Firestore query on the id's I will get from those keys. When I try to retrieve the keys from the real-time database as a string, the string is null here: whereEqualTo("uid", recentUserId). How can I get the keys so that I can perform the query?

String recentUserId;

// Code to get the user id's from the realtime database stored in keys
firebaseDatabase.getReference().child("Recent_Chats").child(auth.getUid()).get().addOnSuccessListener(new OnSuccessListener<DataSnapshot>() {
    @Override
    public void onSuccess(DataSnapshot dataSnapshot) {

        for (DataSnapshot snapshot1 : dataSnapshot.getChildren()) {

            recentUserId = snapshot1.getKey();
            Toast.makeText(getContext(), "hey therere" + recentUserId, Toast.LENGTH_SHORT).show();

        }
    }
});


// Code to get the relevant ids user data from Firestore
Query query = firebaseFirestore.collection("Users").whereEqualTo("uid", recentUserId);
query.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
    @Override
    public void onSuccess(QuerySnapshot queryDocumentSnapshots) {

        List<DocumentSnapshot> list = queryDocumentSnapshots.getDocuments();
        for (DocumentSnapshot d : list) {
            User obj = d.toObject(User.class);
            userArrayList.add(obj);
        }

        adapter.notifyDataSetChanged();
    }
});

Realtime Database Structure:

I tried different ways, but was unable to get keys.

Loading data from Firebase (and most modern cloud APIs) is asynchronous, and the way you have written your code now means that the query on Firestore will happen before the recentUserId = snapshot1.getKey(); has ever been set.

To ensure the correct order, all code that needs data from the database has to be inside the onDataChange that fires when the data is loaded, or be called from there, or be otherwise synchronized.

So the simplest fix is to move the loading from Firestore into the onDataChange like this:

String recentUserId;

// Code to get the user id's from the realtime databse stored in keys
firebaseDatabase.getReference().child("Recent_Chats").child(auth.getUid()).get().addOnSuccessListener(new OnSuccessListener<DataSnapshot>() {
    @Override
    public void onSuccess(DataSnapshot dataSnapshot) {
        for (DataSnapshot snapshot1 : dataSnapshot.getChildren()) {
            recentUserId = snapshot1.getKey();
            Toast.makeText(getContext(), "hey therere" + recentUserId, Toast.LENGTH_SHORT).show();

            // 👇    Code to get the relevent ids user data from firestore
            Query query = firebaseFirestore.collection("Users").whereEqualTo("uid", recentUserId);
            query.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                    List<DocumentSnapshot> list = queryDocumentSnapshots.getDocuments();
                    for (DocumentSnapshot d : list) {
                        User obj = d.toObject(User.class);
                        userArrayList.add(obj);
                    }

                    adapter.notifyDataSetChanged();
                }
            });
        }
    }
});

Also see my longer explanations here: getContactsFromFirebase() method return an empty list and Setting Singleton property value in Firebase Listener