Data is loaded from Firebase asynchronously. Right now your Log.i("asdfads",(bilgiler.get("asdf"))); is run before bilgiler.put("asdf", document.getData().toString()) runs, which explains why you don’t see the value from the database.

Any code that needs data from the database, needs to be inside the onComplete method, or be called from there. So:

db.collection("tarihBilgi")
    .get()
    .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            if (task.isSuccessful()) {
                for (QueryDocumentSnapshot document : Objects.requireNonNull(task.getResult())) {
                    bilgiler.put("asdf", document.getData().toString());
                    Log.i("asdfads",(bilgiler.get("asdf")));
                }
            } else {
                Log.w("TAG", "Error getting documents.", task.getException());
            }
        }
    });

Also see:

• How to check a certain data already exists in firestore or not
• User login in Firebase
• How to return a DocumentSnapShot as a result of a method?
• Wait until Firestore data is retrieved to launch an activity
• Problems with Firestore connections executing threads

I am trying to get data from Firebase to bilgiler variable. But I get error all the time. How can I solve this?

Map<String, String> bilgiler = new HashMap<>();

db.collection("tarihBilgi")
                .get()
                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        if (task.isSuccessful()) {
                            for (QueryDocumentSnapshot document : Objects.requireNonNull(task.getResult())) {
                                //Log.d("TAG", document.getId() + " => " + document.getData());
                                bilgiler.put("asdf", document.getData().toString());
                            }
                        } else {
                            Log.w("TAG", "Error getting documents.", task.getException());
                        }
                    }
                });
        Log.i("asdfads",(bilgiler.get("asdf")));