I've only used the Guava version of Optional, which appears to have a slightly different API than Java 8's. (You can tell there's a Java 8 committee, that's for sure.)

The biggest problem I found in applying Optional is that Optional is very much a Haskell-y concept to me, and Java programmers and Haskell programmers don't really overlap much. Therefore, to me, Optional is just a functor like a list or a set. So doing Optional.asSet() and iterating over the result is, to me, the correct way way of handling an optional value. If there's something in there, the loop runs once. If there's not anything in there, the loop runs zero times. This is exactly like "fmap" in Haskell:

  Prelude Data.Functor> fmap (+1) $ Just 42
  Just 43
  Prelude Data.Functor> fmap (+1) $ Nothing
  Nothing

(Except that Haskell automatically preserves the type safety, whereas you have to re-wrap the value if you're using Java. Functional programming in Java never works, and this where the Optional idea starts to go wrong.)

But I got "I don't really understand this" in code reviews every time I did that; the Java way is to check if it's set, then do a type-unsafe unboxing. Thus, like the article said, completely defeating the purpose of the Optional type.

That said, it's pretty rare to need exactly 0 or 1 of something. If you're doing a database query, you can probably make the API return a set of rows. Then it's natural to loop over the results and nobody raises their eyebrows.