Wsrtjtyk

I am working on a MapActivity where I am getting supplier information from my database and storing it in Supplier object. I am calling two functions fetchSupplier() and FetchStationInfo() where first one fetch supplier info and stores into Object and second one uses it. So, the problem is when I run the fetchSupplier() alone the data gets fetched and stored in my object but, when I call both functions i.e FetchStationInfo() after fetchSupplier() it gives a null pointer exception in FetchStationInfo(). Here's how I am calling the functions:

@Override

public void onMapReady(GoogleMap googleMap) {

    mMap = googleMap;

    fetchSupplier();

    fetchStationInfo();

}

Both of my functions:

private void fetchSupplier(){

    String uid = mAuth.getUid();

    mDatabase = FirebaseDatabase.getInstance().getReference()

            .child("SUPPLIERS").child(uid);



    mDatabase.addListenerForSingleValueEvent(new ValueEventListener() {

        @Override

        public void onDataChange(DataSnapshot dataSnapshot) {

            supplier = dataSnapshot.getValue(Supplier.class);

            Log.i("USER FETCH COMPLETE ","["+supplier.getName()+"]");

        }

        @Override

        public void onCancelled(DatabaseError databaseError) {

            Log.i("FETCHING USER [ERROR] ",databaseError.getMessage());

        }

    });

}



private void fetchStationInfo(){

    //Exception is here in this line where I am calling `getStation_id()`

    mDatabase = FirebaseDatabase.getInstance().getReference()

            .child("STATIONS").child(supplier.getStation_id());



    mDatabase.addListenerForSingleValueEvent(new ValueEventListener() {

        @Override

        public void onDataChange(DataSnapshot dataSnapshot) {

            station = dataSnapshot.getValue(Station.class);

            Log.i("STATION FETCH COMPLETE ","["+station.getName()+"]");

        }

        @Override

        public void onCancelled(DatabaseError databaseError) {

            Log.i("STATION FETCH [ERROR] ",databaseError.getMessage());

        }

    });

}

My objects are declared globally. Been working on it for hours but unable to fix it. My firebase structure:

The problem is that fetchStationInfo() is trying to fetch supplier from fetchSupplier() .

Remember that Firebase calls are asynchronous and you are trying to fetch suppliers while it's being fetched.

So, if you do this

public void onMapReady(GoogleMap googleMap) {

    mMap = googleMap;

    fetchSupplier();

    fetchStationInfo();

}

The two methods will fire and since they are asynchronous you don't know which one ends first, so, if fetchStationInfo ends first it will have suppliers with a null value since fetchSupplier did not finished fetching the result yet.

In order to fix this, you can make a simple interface after fetchSuppliers() that will notify when the job is done, after that job is done , in the callback of that interface you run fetchStationInfo() , in this order you will always have the suppliers info before trying to fetch it in fetchStationInfo().

This is know as a Race Condition.

Please visit these links in order to understand better how it works:

How to return dataSnapshot value as a result of a method?

android - Firebase return value from datasnapshot Why?