stream 分组（group)和map的值映射mapping

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@Data public class MiddleFloorDto {     private Long middleId;     private Long floorId;     private Long landingpageId; }

　　

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public class Main {     public static void main(String[] args) {         ArrayList<MiddleFloorDto> list = new ArrayList<>();           MiddleFloorDto dto1 = new MiddleFloorDto();         dto1.setFloorId(1l);         dto1.setLandingpageId(1l);           MiddleFloorDto dto2 = new MiddleFloorDto();         dto2.setFloorId(12l);         dto2.setLandingpageId(2l);           MiddleFloorDto dto3 = new MiddleFloorDto();         dto3.setFloorId(8l);         dto3.setLandingpageId(2l);           MiddleFloorDto dto4 = new MiddleFloorDto();           dto4.setMiddleId(2l);         dto4.setLandingpageId(3l);            MiddleFloorDto dto5 = new MiddleFloorDto();            dto5.setFloorId(2l);         dto5.setLandingpageId(4l);           MiddleFloorDto dto6 = new MiddleFloorDto();           dto6.setFloorId(4l);         dto6.setLandingpageId(2l);           MiddleFloorDto dto7 = new MiddleFloorDto();           dto7.setFloorId(6l);         dto7.setLandingpageId(2l);         list.add(dto7);         list.add(dto6);         list.add(dto5);         list.add(dto4);         list.add(dto3);         list.add(dto2);         list.add(dto1);          Map<Long, List<MiddleFloorDto>> collect = list.stream().collect(Collectors.groupingBy(MiddleFloorDto::getLandingpageId));          System.out.println(collect.get(1l));         System.out.println(collect.get(2l));         System.out.println(collect.get(3l));         System.out.println(collect.get(4l));         System.out.println(collect.get(5l));         System.out.println(collect.get(6l));      }

　　

list.stream() .collect(Collectors.groupingBy(MiddleFloorDto::getLandingpageId, Collectors.mapping(MiddleFloorDto::getFloorId, Collectors.toList())));

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<br><br>  /* [MiddleFloorDto{middleId=null, floorId=1, landingpageId=1}]            [MiddleFloorDto{middleId=null, floorId=6, landingpageId=2}, MiddleFloorDto{middleId=null, floorId=4, landingpageId=2}, MiddleFloorDto{middleId=null, floorId=8, landingpageId=2}, MiddleFloorDto{middleId=null, floorId=12, landingpageId=2}]            [MiddleFloorDto{middleId=2, floorId=null, landingpageId=3}]            [MiddleFloorDto{middleId=null, floorId=2, landingpageId=4}]            null            null*/<br>测试结果符合预期。