Mysql数据：

代码如下：

Entity：

@Data
@Accessors(chain = true)
public class Region {
    private BigInteger id;
    //名称
    private String name;
    //父id
    private BigInteger parentId;
    private List<Region> children;
    private Integer createTime;
    private Integer updateTime;
    private Integer isDeleted;
}

 Service核心代码：

List<Region> reginaList = regionMapper.findReginaList();

        //将数据以父类ID分组成map：{1:[xxx,xxx],2:[xxx,xxxx]}
        Map<BigInteger, List<Region>> parentIdMap = reginaList.stream().collect(Collectors.groupingBy(Region::getParentId, Collectors.toList()));

        //循环每个数据，从父类MAP中通过他的ID获取他的子类塞到children中
        reginaList.forEach(region -> addChildren(region, parentIdMap));

        //只筛选出最顶级数据
        List<Region> regions =
                reginaList.stream().filter(regina -> regina.getParentId().compareTo(BigInteger.ZERO) == 0).collect(Collectors.toList());


 private void addChildren(Region region, Map<BigInteger, List<Region>> parentIdMap) {
        List<Region> regions = parentIdMap.get(region.getId());
        if (regions != null) {
            region.setChildren(regions);
            regions.forEach(child -> addChildren(child, parentIdMap)); // 递归添加子级的子级
        }
    }

第二种写法：

        reginaList.stream().filter(city-> city.getParentId().compareTo(BigInteger.ZERO) == 0).peek(city -> city.setCityNames(getChildren(city,reginaList)))
  .collect(Collectors.toList());


    public List<Region> getChildren(Region region, List<Region> regionList) {
        List<Region> childrens = regionList.stream().filter(re -> re.getParentId().compareTo(region.getId()) == 0)
                .map(childRegion -> {
                    childRegion.setAddressNames(getChildren(childRegion, regionList));
                    return childRegion;
                }).collect(Collectors.toList());
        return childrens;
    }