Java: When to add readObjectNoData() during serialization?

Question

I am reading the serialization chapter in Effective Java. I am trying to understand the paragraph below, which is found in the book.

If you implemen

Answer 1

"Extendable" means "can have a subclass".

readObjectNoData is used in an unusual case where the serializer (writer) is working with a version of a class with no base class, whereas the deserializer (reader) of the class has a version of the class that IS based on a subclass. The subclass can say "it's ok if my base class isn't in the serialized data - just make an empty one" by implementing readObjectNoData. See these release notes.