"If something is wrong, throw an exception."

That is sometimes just not an option performance wise yet.

"If it's the kind of error that must be handled, throw a checked exception. If there's no value to return, return Optional.empty."

Again, why is Optional.empty() better then null? What makes it better? What do you get from throwing an exception? What is the benifit. You can't just say "my way is better" when we have years and years of Java development flying contrary to your statement. What has changed that makes null a non-viable practice?

"The code receiving that Optional<Foo> is now free to do what the author is suggesting: map, filter, all without worrying if the value is actually there or not. The code is cleaner, easier to read, etc."

The code being cleaner is a subjective, or at least extremely difficult to prove, statment. For instance

List<Integer> nums = getNumbers();
if (nums == null) {
S.o.p("Failed to load numbers");
return;
}
for (Integer i : nums)
System.out.println(i);

Is far better then

Optional<List<Integers>> nums = getNumbers();
if (!nums.isPresent()) {
S.o.p("Failed to load numbers");
return;
}
nums.forEach(S.o::p);

Or even better yet

for (int i : nums)
if (i < 10)
S.o.p(i);

VS

nums.filter((n) -> n < 10).forEach(S.o::p);

I don't think that's more readable. It think that's more compressed. HEre's another example. Suppose we have a magical language that I'm sure you'll pick up. It's a very compressed (or as you'd say expressive) language. This is that same code written in it

pa(i i nums < 10)

Which expands to "print all the ints `i` in nums that are less then 10" in english. That's far less readable. It is more compressed. I don't think compression is the goal of a language as much as it is a goal of a zip program. We're supposed to be writing software, not liturature to submit to the shortest story fully told contest. Readability is a function of correct verbosity.

In my opinion. Just compressing your logic doesn't make it more readable. I think some verbosity is needed to preserve simplicity.

"But no one should ever have to check for null anymore. "

I mean that just doesn't make sense. If you're suggesting that there are some times the state of a program should never contain a null value is just ridiculous. Some things are correctly modled by null and some things are also too performance dependant to not use null.

I think some things benifit from using Optional<> but the case doesn't exist to completely remove null. Even just by the creation of a new container object wrapping your already expensive return object there exists a case for null to exist.

Just saying "Don't do it it's bad" is not proof. Saying "the code is cleaner, easier to read, etc" is not proof or even an example of a design that is simpler to pickup and get going with. You'd have to write some code with the Java/OOP paradigms and write a version (that is feature complete) with the FP paradigms and see which is easier to understand for a new programmer. I'd be hard pressed to belive that the FP implementation would be simpler. Maybe to you and me but no to someone without the domain specific knowladge required to understand what's going on. Even when I use map, zip, and list comprehensions in my python code it scares off some of my coworkers.