java - 尝试从 Firebase 数据库获取数据，但总是获取空值

Question

我试图从数据库中获取某种数据，但它一直给我空值。

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) {

                    }
                });
            }
        });

我调试了我的代码，它显示“金额”为空。我附上了调试的图片:

这是数据库树:

Answer 1

当您对 Firebase 数据库执行查询时，可能会出现多个结果。因此快照包含这些结果的列表。即使只有一个结果，快照也会包含一个结果的列表。

这意味着您的代码需要通过循环 dataSnapshot.getChildren() 来处理快照的列表方面:

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;
            ...