Java8 流 : Collect elements after a condition is met

Question

我的POJO如下

class EventUser {
  private id;
  private userId;
  private eventId;
}

我检索 EventUser 对象如下:

List<EventUser> eventUsers = eventUserRepository.findByUserId(userId);

假设“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"}
]

使用流式传输，并且不使用任何中间变量，如何在给定 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，您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:这只对单线程流有意义。

List<EventUser> filteredByOffSet = 
     eventUsers.stream()
               .filter(from(e -> "id301".equals(e.getId()))
               .collect(Collectors.toList());