import java.util.*;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.stream.Stream;

class Test {
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("w", "o", "l", "f").parallel();
        SortedSet<String> set = stream.collect(ConcurrentSkipListSet::new, Set::add,
                Set::addAll);
        System.out.println(set); // [f, l, o, w]
    }
}