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话不多说直接上代码

//按自定义格式对数据集合List进行分组
//List.stream().collect(Collectors.groupingBy(对象Vo -> {分组自定义标志字段}))
List<TestRecord> TestRecordsByMonthList = mapper.selectTestRecordByCondition(plantId, startDate, endDate);
Map<String, List<TestRecord>> mapGroupByMonth = TestRecordsByMonthList .stream().collect(Collectors.groupingBy(testRecord -> {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM");
    return sdf.format(testRecord.getPlanSamplingTime());
}));

//常规方式，按数据集合List内元素特定字段进行分组
//List.stream().collect(Collectors.groupingBy(对象Vo::分组标志字段))
List<Equipment> equipmentList = operateCenterMapper.selectEquipmentFailureGroupByOrgId(plantId,startDate,endDate);
Map<String, List<Equipment>> mapGroupByOrg = equipmentList.stream().collect(Collectors.groupingBy(Equipment::getOrgId));