[beached_whale]

As nice as it would be to have a method in java that cannot receive a null argument like Result method( Type NOT NULL name ), it isn't there.

I fail to see much difference between an Optional has/doesn't have a value and null. It's just paint and there is no insight into why there is no value. Something like an Expected type that has either a value or the Error/Exception is much more explicit and may let someone do something about it. At least then the user of the method can choose what an appropriate action is with knowledge. But optional and null are the same and give you no more information than a result or that there is no result.

[village-idiot]

Why is map and filter nice on a list? Because the individual functions passed in have no idea they're in the middle of a map or filter. Ditto with mapping or filtering on Optional or Either. It composes better.