Comment by premium-concern

Consider two functions (excuse my Scala):

  val str: Function[String, String] = s => if (s.length > 3) s else null

  val num: Function[String, Integer] = s => if (s == null) -1 else s.length

With Optional, you receive different results depending on whether you call map twice, or combine the functions first:

  scala> Optional.of("Foo").map[String](str).map[Integer](num)
  res12: java.util.Optional[Integer] = Optional.empty

  scala> Optional.of("Foo").map[Integer](str.andThen(num))
  res15: java.util.Optional[Integer] = Optional[-1]

This is incorrect and violates the functor law.