Collector 를 사용하여 groupingBy 하기

List<StatisticsVO> statisticsList = Arrays.asList(logs)

Map<String, Map<String, Integer>> groupData = 

statisticsList.stream()

  .collect(

  Collectors.groupingBy(StatisticsVO::getDateTime, 

  Collectors.groupingBy(StatisticsVO::getUserName

  , Collectors.reducing(0, StatisticsVO::getCount, (int1, int2) -> int1+ int2))));