I am really wondering why this is on HN front-page. It does not provide any good, in-depth analysis of FP in Java 8. In the contrary, the writing seems to be incoherent and assuming knowledge of a certain Java stack.
In particular, I both love and hate the new streams API. It's nice to work with but easily leads to code that is hard to read and hard to debug.
No, I wouldn't call Java 8 a functional language either. Its 'functional' features are more of a tease than anything else, as you'll quickly discover their limitations (and they definitely feel tacked-on rather than an integral part of the language).
Similarly, methods for mutation (compulsory, no less!) are red flag in FP.
So about this setIfNotNull: I don't understand why this method is used in updateEntity. To me it seems the accountType property in the dto can be set with whatever is in the entity, null or not null. And of course, Java 8 has introduced the Optional class as a better way of dealing with null values, so why bother writing a workaround at all?
Yes, it's strange. I'd prefer to map a null in a DTO to an empty optional. I'd like even more to never have nulls in the dto in the first place.