lambda表达是实现递归父节点与子节点的包含关系

/**
 * 商品三级分类
 * 
 * @author zhutong
 * @email 772929030@qq.com
 * @date 2021-08-27 10:40:19
 */
@Data
@TableName("pms_category")
public class CategoryEntity implements Serializable {
	private static final long serialVersionUID = 1L;

	/**
	 * 分类id
	 */
	@TableId
	private Long catId;
	/**
	 * 分类名称
	 */
	private String name;
	/**
	 * 父分类id
	 */
	private Long parentCid;
	/**
	 * 层级
	 */
	private Integer catLevel;
	/**
	 * 是否显示[0-不显示，1显示]
	 */
	private Integer showStatus;
	/**
	 * 排序
	 */
	private Integer sort;
	/**
	 * 图标地址
	 */
	private String icon;
	/**
	 * 计量单位
	 */
	private String productUnit;
	/**
	 * 商品数量
	 */
	private Integer productCount;

	/**
	 * 目录下的子目录
	 */
	@TableField(exist = false) //此注解的意思是表中没有这个字段
	private List<CategoryEntity> children;

}

    /**
     * 查出所有分类以及子分类，以树状结构组装起来
     */
    @RequestMapping("/list/tree")
    //@RequiresPermissions("product:category:list")
    public R list(){
        List<CategoryEntity> categoryEntities = categoryService.listWithTree();

        return R.ok().put("data", categoryEntities);
    }

@Service("categoryService")
public class CategoryServiceImpl extends ServiceImpl<CategoryDao, CategoryEntity> implements CategoryService {

    @Override
    public PageUtils queryPage(Map<String, Object> params) {
        IPage<CategoryEntity> page = this.page(
                new Query<CategoryEntity>().getPage(params),
                new QueryWrapper<CategoryEntity>()
        );

        return new PageUtils(page);
    }

    @Override
    public List<CategoryEntity> listWithTree() {
        List<CategoryEntity> categoryEntities = baseMapper.selectList(null);
        List<CategoryEntity> collect = categoryEntities.stream().filter(categoryEntity -> categoryEntity.getParentCid() == 0) //过滤出顶级节点，就是没有父节点的节点
                .map(categoryEntity -> {
                    categoryEntity.setChildren(getChildren(categoryEntity, categoryEntities));
                    return categoryEntity;
                }).sorted(Comparator.comparingInt(item -> (item.getSort() == null ? 0 : item.getSort()))).collect(Collectors.toList());

        return collect;
    }

    /**
     * 找出父节点的所有子节点，递归实现，
     * 缺点每次都会遍历所有的节点，感觉会影响性能，
     * 后期可能需要算法提高性能
     * @param fatherCategoryEntity  父节点
     * @param all  所有节点
     *
     *             (item1, item2) -> {
     *                     return (item1.getSort() == null ? 0 : item1.getSort()) - (item2.getSort() == null ? 0 : item2.getSort());
     *                 }
     *
     *             Comparator.comparingInt(CategoryEntity::getSort)
     * @return
     */
    private List<CategoryEntity> getChildren(CategoryEntity fatherCategoryEntity,List<CategoryEntity> all){
        List<CategoryEntity> collect = all.stream().filter(categoryEntity -> categoryEntity.getParentCid() == fatherCategoryEntity.getCatId()) //过滤操作，
                .map(categoryEntity -> {
                    categoryEntity.setChildren(getChildren(categoryEntity, all)); //匹配操作，子节点下的字节点
                    return categoryEntity;
                }).sorted(Comparator.comparingInt(item -> (item.getSort() == null ? 0 : item.getSort()))).collect(Collectors.toList()); //排序，lambda的行为参数化

        return collect;
    }