flatMap 保证是懒惰的吗?

I tried with a smaller example and it looks like working as expected. i.e processes data one by one but can this behavior be relied on? If not, does calling .sequential() before .flatMap(...) help?

    Stream.of("one", "two", "three")
            .flatMap(num -> {
                System.out.println("Processing " + num);
                // return FetchFromInternetForNum(num).data().stream();
                return Stream.of(num);
            })
            .peek(num -> System.out.println("Peek before filter: "+ num))
            .filter(num -> num.length() > 0)
            .peek(num -> System.out.println("Peek after filter: "+ num))
            .forEach(num -> {
                System.out.println("Done " + num);
            });

输出:

Processing one
Peek before filter: one
Peek after filter: one
Done one
Processing two
Peek before filter: two
Peek after filter: two
Done two
Processing three
Peek before filter: three
Peek after filter: three
Done three

更新:如果对实施很重要，请使用官方的 Oracle JDK8

 int result = Stream.of(1)
            .flatMap(x -> Stream.generate(() -> ThreadLocalRandom.current().nextInt()))
            .findFirst()
            .get();

    System.out.println(result);

urls.stream()
    .flatMap(url -> fetchDataFromInternet(url).stream())
    .filter(...)
    .findFirst()
    .get();