O TAK

mergeTwoLists.java

    class User {

        public int id;

        public User(int id) {
            this.id = id;
        }
    }

    class Post {

        public Post(Post post, User user) {
            this.id = post.id;
            this.userId = post.userId;
            this.user = user;
        }

        public Post(int id, int userId) {
            this.id = id;
            this.userId = userId;
        }

        public int id;

        public User user;

        public int userId;

    }
    
    ArrayList<User> source = new ArrayList<>();
        source.add(new User(1));
        source.add(new User(2));
        source.add(new User(3));
        source.add(new User(4));

        Observable<User> userObservable = Observable.fromIterable(source);

        ArrayList<Post> source1 = new ArrayList<>();
        source1.add(new Post(1, 1));
        source1.add(new Post(2, 1));
        source1.add(new Post(3, 2));
        source1.add(new Post(4, 2));

        List<Post> posts = Observable.fromIterable(source1)
                .flatMap(post -> userObservable.flatMap(user -> post.userId == user.id ? Observable.just(new Post(post, user)) : Observable.never()))
                .toList()
                .blockingGet();