java1.8從list集合中抽取某個字段轉為新的集合以及把list集合按某個字段進行分組

從list集合中抽取getId轉為新的集合：
List<NatCustomers> natCustomersList = natCustomersService.queryListByPage(0,1000);
List<Integer> customerIds = natCustomersList.stream().map(NatCustomers::getId).collect(Collectors.toList());

把list集合按getCustomerId字段進行分組：

List<ConsumptionStatistics>  consumptionStatistics = natServicesService.getConsumptionStatistics(paramMap);
Map<Integer, List<ConsumptionStatistics>> collect = consumptionStatistics.stream().collect(Collectors.groupingBy(ConsumptionStatistics::getCustomerId));