Collecting lists of optionals to a list containing present optionals

user_965830 0
Viewed 3571

I am trying to refactor some code to return a list of optionals instead of an optional.

I have the following list in my class

private final List<Mapper<? extends Message>> mappers;

There is a private method that creates features for these mappers and returns a list of message

private List<Message> mapToFeature() {
        mappers.stream()
                .map(mapper -> mapper.createFeature())
                .collect(Optionals.toList());
}

The Mapper interface looks like this:

public interface Mapper<T extends Message> {
    Optional<T> createFeature();
}

The Optionals.toList() method returns a Collector to filter present optionals into a List.

I want to change the interface (and all the corresponding classes) to return a list of optionals

public interface Mapper<T extends Message> {
    List<Optional<T>> createFeature();
}

I do not have a method in the Optionals util to filter present options from multiple lists. How would I be able to do the same without making any changes to the util class?

collectors java-stream java-8 java
3 Answers

Collect a stream of Optionals to a list containing only the present values:

List<Optional<String>> someStrings = ...;

List<String> allPresentStrings = someStrings.stream()
    .flatMap(Optional::stream)
    .collect(Collectors.toList());
user_3156770

Since unable to understand your issue properly, it would be better if you will post your util class as well mean while see my go for this:

  private List<Message> mapToFeature() {
       return mappers.stream()
                .map(mapper -> mapper.createFeature())
                .flatMap(List::stream)
                .filter(Optional::isPresent)
                .map(Optional::get)
                .collect(Optionals.toList());
}
user_12452340 
Stream<Optional<T>> present = listOfOptionals.stream().filter(Optional::isPresent);

Stream<T> asTs = present.map(Optional::get); 
// this is safe, because all remaining are present,

//convert Streams to Lists as usual with collect.
List<T> listTs = asTs.collect(Collectors.toList());
user_859310
Related