How to get data from Firebase Database in a loop?

in my code I have a list of products barcode and in my Firebase Database I have values inside this barcodes, like this: .

I'm trying to retrieve the barcodes datas in a loop, but I cannot handle with this. Here is my code:

for (int i = 0; i < listaBarCodeProdutos.size() ; i ++ ){
            String barCode = (String) listaBarCodeProdutos.get(i);
            Query query = produtosRef.orderByKey().equalTo(barCode);
            query.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                        //Limpa o hash map
                        listaMercadosCB.clear();
                        //Salva os mercados e os preços e um hashmap
                        listaMercadosCB = (HashMap) snapshot.child("mercados").getValue();
                        //This is to parse the HashMap keys to Array                         
                        Object[] listaIdsCB = listaMercadosCB.keySet().toArray();


                        //add esse obj em um array de objetos
                        listaCB.add(listaIdsCB);
                        Log.i("listaCB1", String.valueOf(listaCB));
                    }
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });
        }

This code saves all data in barcode child in a HashMap, but when I try to get this HashMap value using Log.i("listaMercadosCB", String.valueOf(listaMercadosCB)); after the loop, it returns null...

This is the logcat for the values: 02-13 19:44:17.146 13903-13903/com.example.mts_rodrigues.projetocartolada I/listaMercadosCB: {wrxYaHZuLBTfrPrrvThxQREFzK02=4.5, d63vvncs2gh6PSnxFAjxhTOayyq1=2.9}

This is inside the loop

02-13 19:44:17.066 13903-13903/com.example.mts_rodrigues.projetocartolada I/listaMercadosCB: {}

This is after de loop

I guess that the value outside the loop is called before the Firebase Database retrieved him inside the loop...

All this codes are inside the onCreate() method. Anyone can help me ? Thanks!