android - can't get values out of ondatachange method

Question

I'm currently developing an android app where I'm using firebase as database but when in got the variable in the onDataChange method and I assign them to a global variables I got null variables but when I call those variables in the onDataChange method they are not null.

public class PositionateMarkerTask extends AsyncTask {
    public ArrayList<Location> arrayList= new ArrayList<>();
    public void connect() {
        //setting connexion parameter
        final Firebase ref = new Firebase("https://test.firebaseio.com/test");
        Query query = ref.orderByChild("longitude");

        //get the data from the DB
        query.addListenerForSingleValueEvent(new ValueEventListener() {

            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                //checking if the user exist
                if(dataSnapshot.exists()){
                    for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
                        //get each user which has the target username
                        Location location =userSnapshot.getValue(Location.class);
                            arrayList.add(location);
                        //if the password is true , the data will be storaged in the sharedPreferences file and a Home activity will be launched
                    }
                }
                else{
                    System.out.println("not found");
                }
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {
                  System.out.println("problem ");

            }
        });
    }

    @Override
    protected Object doInBackground(Object[] params) {
        connect();
        return null;
    }

    @Override
    protected void onPostExecute(Object o) {
        super.onPostExecute(o);

        System.out.println("the firs long is"+arrayList.get(0).getLongitude());

    }
}

Answer 1

Waitting for answers