Why the size of OptoinalXxx.emty() is zero？

[sunriseLe]

• In class SizeOf, I found the size of OptoinalXxx.emty() is zero. In my opinion, the size of empty instance should be OPTIONAL_INSTANCE_SIZE or OPTIONAL_INT_INSTANCE_SIZE.
• Because it's an instance and will use basic memory for class layout

  public static <T> long sizeOf(Optional<T> optional, ToLongFunction<T> valueSize)
  {
      return optional != null && optional.isPresent() ? OPTIONAL_INSTANCE_SIZE + valueSize.applyAsLong(optional.get()) : 0;
  }
  
  
  public static long sizeOf(OptionalInt optional)
  {
      return optional != null && optional.isPresent() ? OPTIONAL_INT_INSTANCE_SIZE : 0;
  }