Locale.writeObject sets and reads the hashcode field without any synchronization. As such, even though the hashCode method is synchronized, the hashCode method could return -1 if a separate thread executing writeObject runs in between the assignment and return statements of the hashCode method. And as such, writeObject would write out a value other than -1 if a separate thread executing hashCode runs in between the assignment and defaultWriteObject statements of the writeObject method.