Firebase ValueEventListener onDataChange() not being called

Question

I've traced my code to the root cause of my crashes and apparently this constructor is unable to update the variables of the class. I am getting null pointer exception when I'm trying to get data from this class. You can safely assume that the record is already in the database and all it has to do is just get the data and place it in the class/object. I simply want to get the name for now because I'm testing if the object is still null or not.

class Saver{
    private String name;
// constructor of Saver class
public Saver(final String uid) {
        Log.d("Saver.java","Reached here"); // this works
        FirebaseDatabase.getInstance().getReference().addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(final DataSnapshot dataSnapshot) {
                Log.d("Saver.java", "OnDataChange"); // does not work
                if(dataSnapshot.hasChild("users/" + uid)){
                    LoadRecord(dataSnapshot, uid);
                }
                else{
                    // set a new record into the database
                    FirebaseDatabase.getInstance().getReference().child("users/" + uid).setValue(CreateNewRecord(FirebaseAuth.getInstance().getCurrentUser())).addOnCompleteListener(new OnCompleteListener<Void>() {
                        @Override
                        public void onComplete(@NonNull Task<Void> task) {
                            if(task.isSuccessful()){
                                LoadRecord(dataSnapshot, uid);
                            }
                            else{
                                Log.e("Saver.java","Failed to set record in database");
                            }
                        }
                    });
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                  Log.e("LifeSaver.java","Error: " + databaseError.getMessage());
            }
        });
    }

// This function loads the data into the object
private void LoadRecord(DataSnapshot dataSnapshot, String uid){
        Log.d("LifeSaver.java","Uid:"+uid);
        // load the existing record in the database
        Saver temp = dataSnapshot.child("users/" + uid).getValue(Saver.class);
        setName(temp.getName());
    }
}
public void setName(String name) {
    this.name = name;
}
private Saver CreateNewRecord(FirebaseUser firebaseUser){
    Saver saver = new Saver ();
    saver.setName(firebaseUser.getDisplayName());
    Log.d("saver","saver name: " + saver.getName());
    return continuLifeUser;
}

Obviously the function onDataChange will not run until something changed on the database.

1. How can I manually trigger this function, if possible?
2. Should I have a DataSnapshot that has the children of this node? If so, how? (show code please so I can visualize what you are explaning)
3. Is there a better way of doing this?

---

Edit 1: Here's the logcat statements from the constructor that should have called the FirebaseDatabase:

D/Saver.java: Reached here with hA4hZrBieISwMOZaMYe7m6K5tpI3
I/DynamiteModule: Considering local module com.google.android.gms.firebase_database:4 and remote module com.google.android.gms.firebase_database:6
I/DynamiteModule: Selected remote version of com.google.android.gms.firebase_database, version >= 6
I/art: DexFile_isDexOptNeeded failed to open oat file '/data/dalvik-cache/x86_64/data@data@com.google.android.gms@app_chimera@m@00000004@DynamiteModulesC_GmsCore_prodlmp_alldpi_release.apk@classes.dex' for file location '/data/data/com.google.android.gms/app_chimera/m/00000004/DynamiteModulesC_GmsCore_prodlmp_alldpi_release.apk': Failed to open oat filename for reading: No such file or directory
D/Saver.java: Ended here
D/LoginAct.java: Name: null

Answer 1

For getting all the names from all users under users node, please use the following code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference usersRef = rootRef.child("users");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String name = ds.child("name").getValue(String.class);
            Log.d("TAG", name);
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
usersRef.addListenerForSingleValueEvent(eventListener);

The output will be:

Michael Ong
//other names

If you want to get only the name of a particular user, please use the following code:

FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
String uid = firebaseUser.getUid();

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference uidRef = rootRef.child("users").child(uid);
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        String name = dataSnapshot.child("name").getValue(String.class);
        Log.d("TAG", name);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
uidRef.addListenerForSingleValueEvent(eventListener);

The output will be only a single record:

Michael Ong

Answer 2

I resolved the problem by having a loading screen. I should have remembered that the call was Async and that it needed a receiver to call the function that will place the data before moving on to the next activity.

private interface OnGetDataListener {
    void onSuccess(DataSnapshot dataSnapshot);
    void onFailure();
}

Here's a small code for the listener that you'll have to implement when you're calling the database to get the data. You can check for other posts similar to this one.

Answer 3

Hi first you should add data to database , then only it will work . Looks like there is no data available in Firebase.Just add a key value pair in firebase database , then it will be triggered.

FirebaseDatabase.getInstance().getReference().child("users/" + uid).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(final DataSnapshot dataSnapshot) {
            Log.d("Saver.java", "OnDataChange"); // does not work
            if(dataSnapshot!=null){
                LoadRecord(dataSnapshot);
            }
            else{
                // set a new record into the database
                FirebaseDatabase.getInstance().getReference().child("users/" + uid).setValue(CreateNewRecord(FirebaseAuth.getInstance().getCurrentUser())).addOnCompleteListener(new OnCompleteListener<Void>() {
                    @Override
                    public void onComplete(@NonNull Task<Void> task) {
                        if(task.isSuccessful()){
                            LoadRecord(dataSnapshot);
                        }
                        else{
                            Log.e("Saver.java","Failed to set record in database");
                        }
                    }
                });
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
              Log.e("LifeSaver.java","Error: " + databaseError.getMessage());
        }
    });
}
   // Load record should be like this...
   private void LoadRecord(DataSnapshot dataSnapshot){
    Log.d("LifeSaver.java","Uid:"+uid);
    // load the existing record in the database
    Saver temp = dataSnapshot.getValue(Saver.class);
    setName(temp.getName());
}

}