从Firebase获取唯一ID的子节点的值

Question

我试图从实时数据库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onMapReady中使用此代码从Firebase获取位置时获得npe，我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

您将得到以下错误:

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

因为你在代码中使用了不正确的引用。在我看来，这两个"纬度"one_answers";longitude"是UID的直接子节点。因此，请更改以下代码行:

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

:

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

其余代码可以保持不变。

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");

这里你已经保存了用户信息

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

然后你进入"用户"节点进一步。但是没有"用户位置"在"Users"节点。

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

你提供了错误的ref path，这就是为什么你得到NPE。

转换为:

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

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