从java中的arraylist中删除所有可选的空/空值

Question

I need to remove all empty/null values from List<Optional<String>> .

Example:

List<Optional<String>> list = new ArrayList<>();

list.add(Optional.empty());
list.add(Optional.of("Str1"));
list.add(Optional.of("Str2"));
list.add(Optional.of("Str3"));
list.add(Optional.of("Str4"));
list.add(Optional.of("Str5"));
list.add(Optional.empty());
list.add(Optional.ofNullable(null));

---

Currently, I'm using one of the below approaches:

Way 1:

List<String> collect = list.stream()
                   .filter(Optional::isPresent)
                   .map(obj ->obj.get())
                   .collect(Collectors.toList());

Way 2:

List<Optional<String>> emptlist = new ArrayList<>();
emptlist.add(Optional.empty());
list.removeAll(emptlist);

Is there any other better way?

Answer 1

With Java9 , you can do this using the newly added Optional::stream API :

List<String> collect = list.stream()
               .flatMap(Optional::stream)
               .collect(Collectors.toList());

This method can be used to transform a Stream of optional elements to a Stream of present value elements .

Sticking with Java8 , the Way1 in the question is good enough IMHO -

List<String> collect = list.stream()
               .filter(Optional::isPresent)
               .map(Optional::get) // just a small update of using reference
               .collect(Collectors.toList());

Answer 2

removeIf是最简单的方法：

list.removeIf(x -> !x.isPresent());

Answer 3

For anyone who wants to have a little fun or understand working of Optionals and Streams better (Java 8):

List<String> collect=list.stream().
                                   map(z->z.map(Stream::of)).
                                   flatMap(ox->ox.orElseGet(Stream::empty)).
                                   collect(Collectors.toList());

1. The first map function converts Optional[String] to Optional [Stream[String]]. A Stream String[Optional [Stream[String]]] is thus passed forward
2. The "map" part of flatmap converts each Optional[Stream[String]] to Stream[String]. All empty Optionals are replaced by an empty Stream. Now we have Stream[Stream[String]].
3. The "flat" part converts Stream[Stream[String]] to Stream[String].