#vulnerable code 
                                public Iterator<Marker> iterator() {
        if (referenceList != null) {
            return referenceList.iterator();
        } else {
            List<Marker> emptyList = Collections.emptyList();
            return emptyList.iterator();
        }
    } 
                            #location 3 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                public Iterator<Marker> iterator() {
      return referenceList.iterator();
    }