使用orderByChild查询从Firebase获取数据 [英] Getting data from Firebase using orderByChild query

问题描述

我有一个字符串stationName，我想根据我提供的stationName从FireBase获取距离. 这是我的Firebase数据库:

I have a String stationName and I want to fetch distance from FireBase according to the stationName I provided. Here is my Firebase Database:

我正在使用orderByChild查询来过滤结果，我只需要获取距离值.这是我的代码:

I am using orderByChild query to filter result and I need fetch only the distance value. Here is my code:

Query query = FirebaseDatabase.getInstance().getReference("Stations").orderByChild("Name").limitToFirst(1).equalTo(sourceName);
query.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        String distance;
        for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
            distance = dataSnapshot.child("Distance").getValue(String.class);
            tt.setText(distance); //to check if it is working or not
        }
    }
    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {

    }
});

我需要知道代码有什么问题.它没有返回任何值. 预先感谢

I need to know what is wrong with code. It's not returning any value. Thanks in advance

推荐答案

替换此

for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
            distance = dataSnapshot.child("Distance").getValue(String.class);
            tt.setText(distance); //to check if it is working or not
        }

有了这个

for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
            distance = snapshot.child("Distance").getValue().toString(); // you 
            //cannot pass the String.class as in the getValue() parameters becasue a long value 
            //can not be cast to class object so you need to call toString() method to convert long value 
            //into the string value.
            tt.setText(distance); //to check if it is working or not
        }

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