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"}
]

使用流式传输,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:ex:

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

首先查找搜索项的索引:

int asInt = IntStream.range(0, list.size())
    .filter(userInd-> list.get(userInd).equals(<criteria>))
    .findFirst()
    .getAsInt();

获取索引上和之后的项目:

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