val items = Arrays.asList("a", "b", "c")
// doesn't compile because it is Stream<String?>!
val stream: Stream<String> = items
    .stream()
    .map { item ->
       // call that returns nullable, i.e. String?
    }
    .filter(Objects::nonNull)

items.filterNotNull().stream()

!!

val stream: Stream<String> = items
            .stream()
            .filter(Objects::nonNull)
            .map { it!! }