Cant add elements to Arraylist that I'm getting from firebase

Question

I was trying to take the all the key values as String and add them to "studentrolls" Arraylist and trying to return the Arraylist so that I can make Multiple choice Dialog box. But somehow its not adding to any element to the arraylist. Here's my code:

public ArrayList < String > getAllstudents(String coursename) {
  rootnode = FirebaseDatabase.getInstance();
  String uniqueid = coursename + ID;
  reference = rootnode.getReference().child("Course").child(uniqueid).child("students");
  studentrolls = new ArrayList < String > ();
  Log.d(TAG, "Teachers ID:  " + ID);
  reference.addValueEventListener(new ValueEventListener() {

    @SuppressLint("LongLogTag")@Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
      Log.d(TAG, "onDataChange: " + snapshot + "");
      Log.d(TAG, "onDataChange: " + snapshot.getValue() + "");
      Log.d(TAG, "Checking snapshot: " + snapshot.getChildren().toString());
      for (DataSnapshot ds: snapshot.getChildren()) {

        Log.d(TAG, "Students roll:    " + "" + ds.getKey());
        Log.d(TAG, "Data type" + ds.getKey().getClass().getName());
        studentrolls.add(ds.getKey());

        //Log.d(TAG, "Teachers ID from firebase: " + teID);
      }

    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
      Log.d(TAG, "Fetching data was failed");

    }
  });
  Log.d(TAG, "Inside mydatabae helper array list=====" + studentrolls.size());;

  return studentrolls;
}