code: private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
  
          java.io.ObjectOutputStream.PutField fields = s.putFields();
          fields.put("rnd", U.getLong(Thread.currentThread(), SEED));
          fields.put("initialized", true);
          s.writeFields();
      }
docs: Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).
  @param s the stream
  @throws java.io.IOException if an I/O error occurs