如何用RxJava2将多个单词组合成一个句子？

import io.reactivex.Observable;
import io.reactivex.Observer;
import io.reactivex.disposables.Disposable;

import java.util.Arrays;
import java.util.List;

public class RxJavaExample {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("Hello", "World", "RxJava2");

        Observable.create(emitter -> {
            for (String word : words) {
                emitter.onNext(word);
            }
            emitter.onComplete();
        })
        .flatMap(word -> Observable.just(word))
        .toList()
        .map(wordList -> {
            StringBuilder sentence = new StringBuilder();
            for (String word : wordList) {
                sentence.append(word).append(" ");
            }
            return sentence.toString().trim();
        })
        .subscribe(new Observer<String>() {
            @Override
            public void onSubscribe(Disposable d) {
                // 可选实现
            }

            @Override
            public void onNext(String sentence) {
                System.out.println("Combined sentence: " + sentence);
            }

            @Override
            public void onError(Throwable e) {
                // 可选实现
            }

            @Override
            public void onComplete() {
                // 可选实现
            }
        });
    }
}