从 Firebase 获取具有唯一 ID 的子项的值

Question

我正在尝试从实时数据库中显示用户的当前位置，但它没有显示在地图上，数据库的结构如下：

{
  "Blocked Users" : {
    "RCX2HZXIwlSmMHFgDytf1DgZBgi2" : 0
  },
  "User Location" : {
    "RCX2HZXIwlSmMHFgDytf1DgZBgi2" : {
      "latitude" : 37.4035483,
      "longitude" : -122.097715
    }
  },
  "Users" : {
    "RCX2HZXIwlSmMHFgDytf1DgZBgi2" : {
      "Alert Level" : "High",
      "Emergency Type" : "Natural Disaster",
      "address" : "Lapaz Tarlac",
      "emergencyNum" : "0981232387346",
      "name" : "Rafael Campos",
      "phoneNum" : "0981233445675"
    }
  }
}

我在使用 onMapReady 中的代码从 Firebase 获取位置时遇到 NPE，我应该如何解决这个问题？

@Override
    public void onMapReady(@NonNull GoogleMap googleMap) {
        mMap = googleMap;

        DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference("Users");
        DatabaseReference uidRef = rootRef.child("User Location");

        uidRef.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
            @Override
            public void onComplete(@NonNull Task<DataSnapshot> task) {
                if (task.isSuccessful()) {
                    DataSnapshot snapshot = task.getResult();
                    Double latitude = snapshot.child("latitude").getValue(Double.class);
                    Double longitude = snapshot.child("longitude").getValue(Double.class);
                    LatLng location = new LatLng(latitude, longitude);
                    mMap.addMarker(new MarkerOptions().position(location).title(getCompleteAddress(latitude, longitude)));
                    mMap.moveCamera(CameraUpdateFactory.newLatLng(location));
                    mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(location,14F));
                    Log.d("TAG", "latitude, longitude");
                } else {
                    Log.d("TAG", task.getException().getMessage()); //Don't ignore potential errors!
                }
            }
        });


    }

这是堆栈跟踪：

2021-10-23 11:19:19.139 12376-12376/com.example.rescuealertadmin E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.rescuealertadmin, PID: 12376
    java.lang.NullPointerException: Attempt to invoke virtual method 'double java.lang.Double.doubleValue()' on a null object reference
        at com.example.rescuealertadmin.RetrieveMapsActivity$4.onComplete(RetrieveMapsActivity.java:202)
        at com.google.android.gms.tasks.zzi.run(com.google.android.gms:play-services-tasks@@17.2.1:1)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

Answer 1

您收到以下错误：

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“double java.lang.Double.doubleValue()”

因为您在代码中使用了不正确的引用。如我所见，“纬度”和“经度”都是 UID 的直接子级。所以请更改以下代码行：

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference("Users");
DatabaseReference uidRef = rootRef.child("User Location");
uidRef.get().addOnCompleteListener(/* ... /*);

收件人：

String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference uidRef = rootRef.child("User Location").child(uid);
uidRef.get().addOnCompleteListener(/* ... /*);

其余代码可能保持不变。

Answer 2

您的User Location 路径有一个用户列表，因此您的onComplete 需要通过循环遍历结果来处理DataSnapshot 也有一个子节点列表的事实：

uidRef.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DataSnapshot> task) {
        if (task.isSuccessful()) {
            DataSnapshot snapshot = task.getResult();
            for (DataSnapshot child: snapshot.getChildren()) {
                Double latitude = child.child("latitude").getValue(Double.class);
                Double longitude = child.child("longitude").getValue(Double.class);
                LatLng location = new LatLng(latitude, longitude);
                ...
    }
});

Answer 3

您在错误的节点上查找数据。

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference("Users");

这里您保存了用户信息

DatabaseReference uidRef = rootRef.child("User Location");

然后您将进一步进入“用户”节点。但是“用户”节点中没有“用户位置”。

如果你有 UID，你可以得到这样的数据

DatabaseReference uidRef = FirebaseDatabase.getInstance().getReference("User Location").child(UID);

Answer 4

您提供了错误的参考路径，这就是您获得 NPE 的原因。

转换自：

DatabaseReference rootRef = 
FirebaseDatabase.getInstance().getReference("Users");
DatabaseReference uidRef = rootRef.child("User Location");
uidRef.get().addOnCompleteListener(/* ... /*);

到

String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
DatabaseReference rootRef = 
FirebaseDatabase.getInstance().getReference();
DatabaseReference uidRef = rootRef.child("User Location").child(uid);
uidRef.get().addOnCompleteListener(//);