Stream流的方式递归出树形结构

遇到构建菜单，构建树形结构，数据库一般就使用父id来表示，为了降低数据库的查询压力，我们可以使用Java8中的Stream流一次性把数据查出来，然后通过流式处理

实体类

@Data
@Builder
public class Menu {
    /**
     * id
     */
    public Integer id;
    /**
     * 名称
     */
    public String name;
    /**
     * 父id ，根节点为0
     */
    public Integer parentId;
    /**
     * 子节点信息
     */
    public List<Menu> childList;


    public Menu(Integer id, String name, Integer parentId) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
    }

    public Menu(Integer id, String name, Integer parentId, List<Menu> childList) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
        this.childList = childList;
    }
    
}

递归组装树形结构：

 

@Test
    public void testtree(){
        //模拟从数据库查询出来
        List<Menu> menus = Arrays.asList(
                new Menu(1,"根节点",0),
                new Menu(2,"子节点1",1),
                new Menu(3,"子节点1.1",2),
                new Menu(4,"子节点1.2",2),
                new Menu(5,"根节点1.3",2),
                new Menu(6,"根节点2",1),
                new Menu(7,"根节点2.1",6),
                new Menu(8,"根节点2.2",6),
                new Menu(9,"根节点2.2.1",7),
                new Menu(10,"根节点2.2.2",7),
                new Menu(11,"根节点3",1),
                new Menu(12,"根节点3.1",11)
        );

        //获取父节点
        List<Menu> collect = menus.stream().filter(m -> m.getParentId() == 0).map(
                (m) -> {
                    m.setChildList(getChildrens(m, menus));
                    return m;
                }
        ).collect(Collectors.toList());
        System.out.println("-------转json输出结果-------");
        System.out.println(JSON.toJSON(collect));
    }

    /**
     * 递归查询子节点
     * @param root  根节点
     * @param all   所有节点
     * @return 根节点信息
     */
    private List<Menu> getChildrens(Menu root, List<Menu> all) {
        List<Menu> children = all.stream().filter(m -> {
            return Objects.equals(m.getParentId(), root.getId());
        }).map(
                (m) -> {
                    m.setChildList(getChildrens(m, all));
                    return m;
                }
        ).collect(Collectors.toList());
        return children;
    }