java目录树构建

递归创建目录树，实现方法的一种有其他实现可以评论出来

domain层

public class Category {

    private Long id;
    private String name;
    private Long parentId;

    public Category(Long id, String name) {
        this.id = id;
        this.name = name;
    }

    public Category(Long id, String name, Long parentId) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
    }

    public Category() {
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Long getParentId() {
        return parentId;
    }

    public void setParentId(Long parentId) {
        this.parentId = parentId;
    }
}

dto

public class CategoryDto {

    private Long id;
    private String name;
    private Long parentId;
    private Set chilen;


    public CategoryDto(Long id, String name, Long parentId, Set chilen) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
        this.chilen = chilen;
    }

    public CategoryDto(Long id, String name, Long parentId) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
    }

    public CategoryDto(Long id, Long parentId, Set chilen) {
        this.id = id;
        this.parentId = parentId;
        this.chilen = chilen;
    }

    public CategoryDto(Category category){
        this.id = category.getId();
        this.parentId = category.getParentId();
        this.name = category.getName();
    }

    public CategoryDto() {

    }

    public void addSubNode(CategoryDto subNode) {
        if (this.chilen == null) {
            this.chilen = new HashSet<>();
        }

        this.chilen.add(subNode);
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Long getParentId() {
        return parentId;
    }

    public void setParentId(Long parentId) {
        this.parentId = parentId;
    }

    public Set getChilen() {
        return chilen;
    }

    public void setChilen(Set chilen) {
        this.chilen = chilen;
    }
}

测试

public class GetTree {
    public static void main(String[] args) {
        //模拟 一次select查询出来的集合 
        List categories = new ArrayList<>();
        categories.add(new Category(1L,"一级目录1"));
        categories.add(new Category(2L,"一级目录2"));
        categories.add(new Category(3L,"一级目录3"));
        categories.add(new Category(4L,"一级目录4"));
        categories.add(new Category(5L,"一级目录5"));
        categories.add(new Category(7L,"二级目录2-1",2L));
        categories.add(new Category(8L,"二级目录3-1",3L));
        categories.add(new Category(6L,"二级级目录1-1",1L));
        categories.add(new Category(9L,"二级目录3-2",3L));
        categories.add(new Category(10L,"三级目录7-10",7L));
        categories.add(new Category(11L,"三级目录9-11",9L));
        categories.add(new Category(12L,"四级目录11-12",11L));
        CategoryDto root = new CategoryDto();
        //设置根节点 默认写入的一个根节点 看实际业务场景可以改变
        root.setId(0L);
        root.setName("root");
        HashMap nodehMap = new HashMap<>();
        nodehMap.put(0L,root);
        //遍历 如果没有父id 将根节点id 设置为父id
        for (Category category : categories) {
            if (category.getParentId() == null){
                    category.setParentId(0L);
            }
            // 放入到map集合中
         nodehMap.put(category.getId(),new CategoryDto(category));
        }
        // 遍历map集合 
        for (CategoryDto value : nodehMap.values()) {
            // 取出当前的 value 的上级目录
            CategoryDto parent = nodehMap.get(value.getParentId());
            if (parent != null){
                // 存在 将他加入进去
                parent.addSubNode(value);
            }
        }
        // 返回的root 就是一个树形结构
        // return root

    }
}

后期优化思想，目录过多时每次build一次目录很麻烦，如果其他地方用到这个目录的话可以设置为一个对象，先遍历该对象，没有再去调用目录树的方法。