Java8 Stream：满足条件后收集元素

Question

我的POJO如下class EventUser {  private id;  private userId;  private eventId;}我按如下方式检索 EventUser 对象：List<EventUser> eventUsers = eventUserRepository.findByUserId(userId);说'eventUsers'如下：[{"id":"id200","userId":"001","eventId":"1010"},{"id":"id101","userId":"001","eventId":"4212"},{"id":"id402","userId":"001","eventId":"1221"},{"id":"id301","userId":"001","eventId":"2423"},{"id":"id701","userId":"001","eventId":"5423"},{"id":"id601","userId":"001","eventId":"7423"}]使用流，而不使用任何中间变量，如何在给定的 EventUser.id 之后过滤和收集事件：例如：List<EventUser> filteredByOffSet = eventUsers.stream.SOMEFILTER_AND_COLLECT("id301");结果应该是：[{"id":"id301","userId":"001","eventId":"2423"},{"id":"id701","userId":"001","eventId":"5423"},{"id":"id601","userId":"001","eventId":"7423"}]

Answer 1

在 Java 8 中你需要一个有状态的过滤器

public static <T> Predicate<T> from(Predicate<T> test) {

    boolean[] found = { false };

    // once found, always true

    return t -> found[0] || (found[0] = test.test(t));

}

注意：这仅对单线程流有意义。

List<EventUser> filteredByOffSet = 

     eventUsers.stream()

               .filter(from(e -> "id301".equals(e.getId()))

               .collect(Collectors.toList());

Answer 2

使用Java 9 中的“ dropWhile ”。

Answer 3

先找到搜索项的索引：

int asInt = IntStream.range(0, list.size())

    .filter(userInd-> list.get(userInd).equals(<criteria>))

    .findFirst()

    .getAsInt();

获取索引前后的项目：

list.stream().skip(asInt).collect(Collectors.toList());