gistfile1.txt

public class Playground {


  Observable<DiscussionPost> getPost(String commentId) { // this is your Api interface
    return Observable.just(new DiscussionPost());
  }

  Observable<List<DiscussionPost>> getPosts() {
    return getPost("root").flatMap(new Func1<DiscussionPost, Observable<DiscussionPost>>() {
      @Override
      public Observable<DiscussionPost> call(DiscussionPost discussionPost) {
        return Observable.from(discussionPost.posts);
      }
    }).flatMap(new Func1<DiscussionPost, Observable<DiscussionPost>>() {
      @Override
      public Observable<DiscussionPost> call(DiscussionPost discussionPost) {
        return getPost(discussionPost.commentId);
      }
    }).toList();
  }


  static class DiscussionPost {

    String commentId;
    List<DiscussionPost> posts;
  }
}