How to Convert a user defined object to byte array

Question

I am trying to convert an object called lineId that defined by me to byte array in order to feed it back to OutputStream.write

static void serialize(IACLineSegment objectToSerialze, OutputStream outputStream) {
    ConductingEquipmentSerializer.serialize(objectToSerialze, outputStream);
    byte[] bytes
    ID lineId = objectToSerialze.getLine().getId();
    if (lineId != null){
      outputStream.write(//lineId byte array);
      }