firebase return onDataChange data-

public ArrayList< Event > getAllEventsOnFirebase(final EventListAdapter adapter, 
                                                 final ArrayList<Event> events) {
  DatabaseReference eventsTable = getDatabaseTableWith(Constants.tableEvents);
  eventsTable.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
            String eventId = postSnapshot.child(Constants.taEventUUID).getValue(String.class);
            String eventName = event.child(Constants.taEventName).getValue(String.class);
            String eventAddress = event.child(Constants.taEventAddress).getValue(String.class);

            Event newEvent = new Event(eventId, eventName, eventAddress);

            events.add(newEvent);
        }
        adapter.notifyDataSetChanged();
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException();
    }
  });
}

//call with
ArrayList< Event > events = new ArrayList<Event>();
EventListAdapter adapter = new EventListAdapter(this, eventsArray);
eventsListView.setAdapter(adapter);

FirebaseConnection.getInstance().getAllEventsOnFirebase(adapter, events);