从 Firebase 获取唯一 ID 的子代值

Question

I'm trying to display the user's current location from the Realtime Database, but it's not displaying on the map, the Database is structured like this:

 { "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" } } }

I'm getting NPEs when fetching the location from Firebase using this code in onMapReady, how should I fix this?

@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!
                }
            }
        });


    }

This is the stack trace:

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

Your User Location path has a list of users, so your onComplete needs to handle the fact that the DataSnapshot has a list of child nodes too by looping over the results:

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 2

You are finding data on the wrong node.

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

Here you have saved Users Information

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

And then you're going into "Users" Node Further. But there is NO "User Location" In "Users" Node.

You can get data like this if you have UID

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

Answer 3

You are getting the foll9ow error:

java.lang.NullPointerException: Attempt to invoke virtual method 'double java.lang.Double.doubleValue()' on a null object reference

Because you are using an incorrect reference in your code. As I see, both "latitude" and "longitude" are direct children of the UID. So please change the following lines of code:

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

To:

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

The rest of the code may remain unchanged.

Answer 4

You are providing wrong ref path that's why you are getting NPE.

Convert it from:

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

to

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