How to check a specific key is present in the documents of the collection

gistfile1.txt

 private void loadRecentWords() {
        db.collection("TwordColl")
                .orderBy("time", Query.Direction.DESCENDING)
                .limit(10)
                .addSnapshotListener(new EventListener<QuerySnapshot>() {
                    @Override
                    public void onEvent(@Nullable QuerySnapshot value,
                                        @Nullable FirebaseFirestoreException e) {
                        if (e!= null){
                            Log.w(TAG, "listen failed "+e);
                            return;
                        }
                        if (value.isEmpty()){
                            mMarkedWorkListLive.setValue(null);
                        }else {
                            List<WordModel> wordList = new ArrayList<>();
                            for (QueryDocumentSnapshot doc : value){
                                String wo = doc.getString("wo");
                                String tr = doc.getString("tr");
                                long languageCheck=0;
//                                if (!TextUtils.isEmpty(doc.getLong("lan").toString())){
//                                    languageCheck = doc.getLong("lan");
//                                }
                                if (doc.get("lan")!= null){
                                    languageCheck = doc.getLong("lan");
                                    Log.i(TAG, "the language is "+languageCheck);
                                }

                                Log.i(TAG, "wo is "+wo +"  tr is "+tr);
                                boolean mark = doc.getBoolean("mark");
                                wordList.add(new WordModel(wo, tr, mark, languageCheck));
                            }
                            mRecentWordListLive.setValue(wordList);
                        }

                    }
                });
    }

gistfile2.txt

//this query is used to check if key is available, if yes then get value of this key. 
if (doc.get("lan")!= null){
}