Get all the fields of a document

How can I get all the fields of a Firestore document. For example if in a collection I have several documents, and in each document I have fields; but each of these documents does not have the same number of fields, some have more and others less, even those fields are different in each document. I'm trying to get those fields in a list but I can not use getString (), since I do not know what fields a document has at any given moment.

fsdbData.collection("class")
            .orderBy("nom")
            .addSnapshotListener(new EventListener<QuerySnapshot>() {
                @Override
                public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {
                    if (e != null) {
                        Log.d("Ex", "Excepcion", e);
                        return;
                    }
                    if (queryDocumentSnapshots.size() != 0) {
                        lisClases.clear();
                        adaClases.notifyDataSetChanged();
                    }
                    for (QueryDocumentSnapshot doc : queryDocumentSnapshots) {
                        doc.
                        lisClases.add(new ItemClase(
                                doc.getString("nom"),

                        ));