0、先来一个简单的，先分组再聚合

Map<String, Double> categoryMap = baseLevelData.stream().collect(Collectors.groupingBy(o -> o.getCategoryL2(), Collectors.summingDouble(o -> o.getBudgetOk2ship())));

1.对象二级 group

Map<String, Map<Integer, List<WoiTargetAlertRuleEntity>>> woiMap = allRuleEntity.stream().collect(Collectors.groupingBy(o -> o.getCountryCode(), Collectors.groupingBy(o -> o.getWoiTarget())));

2. 二级 group，再mapping属性

Map<String, Map<Integer, List<String>>> secondLevelMap = allRuleEntity.stream().collect(Collectors.groupingBy(o -> o.getCountryCode(), Collectors.groupingBy(o -> o.getWoiTarget(), Collectors.mapping(o -> o.getTerritory(), Collectors.toList()))));

3. group，再取每组的第一条数据

Map<String, ScMbgCalendarCmlEntity> quarterMap = calendarHeader.stream().collect(Collectors.groupingBy(ScMbgCalendarCmlEntity::getMfgQtrDesc, Collectors.collectingAndThen(Collectors.toList(), list -> list.get(0))));

4. group，再 toMap

Map<String, Map<String, String>> map = productList.stream().collect(Collectors.groupingBy(o -> o.getPmdProgramName(), Collectors.toMap(NPIBRSPMDProgramEntity::getPmdProductName, NPIBRSPMDProgramEntity::getPmdProductId)));