Java Stream - 如何将字符串列表减少为单个字符串

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

public class Main {

  public static void main(final String[] args) {
    List<String> stringCollection = new ArrayList<>();
    stringCollection.add("ddd2");
    stringCollection.add("aaa2");
    stringCollection.add("bbb1");
    stringCollection.add("aaa1");
    stringCollection.add("bbb3");
    stringCollection.add("ccc");
    stringCollection.add("bbb2");
    stringCollection.add("ddd1");

    Optional<String> reduced =
            stringCollection
                    .stream()
                    .sorted()
                    .reduce((s1, s2) -> s1 + "#" + s2);
    reduced.ifPresent(System.out::println);

  }

}