试图从 Firebase 数据库中获取数据，但它总是得到 null 值

Question

I'm trying to get some kind of data from the database but it keeps giving me null value.

verifyPromo.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String promoCodeString = promoCode.getEditText().getText().toString();
                Query query = FirebaseDatabase.getInstance().getReference().child("promoCodes").orderByChild("promoCode").equalTo(promoCodeString);
                query.addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        if (dataSnapshot.exists() & promoUnapplied) {
                            Map<String, String> promoCodeInfo = (Map<String, String>) dataSnapshot.getValue();
                            String amount = promoCodeInfo.get("amount");
                            double bill = Double.valueOf(theBillTA.getText().toString());
                            double billAfterDis = bill * Double.valueOf(amount) / 100;
                            bill -= billAfterDis;
                            theBillTA.setText(String.valueOf(bill));
                            promoUnapplied = false;
                            Toast.makeText(NewAppointmentTA.this, "Grate,You Got A Discount", Toast.LENGTH_SHORT).show();
                        } else if (dataSnapshot.exists() & !promoUnapplied) {
                            Toast.makeText(NewAppointmentTA.this, "You Already Get A Discount", Toast.LENGTH_SHORT).show();
                        } else {
                            Toast.makeText(NewAppointmentTA.this, "The Promo Code you Entered Is Not Exist", Toast.LENGTH_SHORT).show();
                        }
                    }
                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {

                    }
                });
            }
        });

I debugged my code and it showed me that the "amount" is null. I included a picture of the debugging:

And here is the database tree:

Answer 1

When you execute a query against the Firebase Database, there will potentially be multiple results. So the snapshot contains a list of those results. Even if there is only a single result, the snapshot will contain a list of one result.

That means your code needs to handle the list-aspect of the snapshot by looping over dataSnapshot.getChildren() :

Query query = FirebaseDatabase.getInstance().getReference().child("promoCodes").orderByChild("promoCode").equalTo(promoCodeString);
query.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
      if (promoUnapplied) {
        for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
            String amount = snapshot.child("amount").getValue(String.class);
            double bill = Double.valueOf(theBillTA.getText().toString());
            double billAfterDis = bill * Double.valueOf(amount) / 100;
            bill -= billAfterDis;
            ...