java 构建目录树

1. 准备 Menu 对象

public class Menu {

    private Integer id;
    private String label;
    private Integer orderNum;
    private Integer pid;
    private List<Menu> children;

    public Menu(Integer id, String label, Integer orderNum, Integer pid) {
        this.id = id;
        this.label = label;
        this.orderNum = orderNum;
        this.pid = pid;
    }

    public List<Menu> getChildren() {
        return children;
    }

    public void setChildren(List<Menu> children) {
        this.children = children;
    }

    public Integer getId() {
        return id;
    }

    public String getLabel() {
        return label;
    }

    public Integer getOrderNum() {
        return orderNum;
    }

    public Integer getPid() {
        return pid;
    }

    @Override
    public String toString() {
        return "Menu{" +
                "id=" + id +
                ", label='" + label + '\'' +
                ", orderNum=" + orderNum +
                ", pid=" + pid +
                ", children=" + children +
                '}';
    }
}

2. 构建MenuList 数据

public class MenuList {
    public static final List<Menu> MENU_LIST=new ArrayList<>();

    static {
        // 一级菜单
        MENU_LIST.add(new Menu(1,"1菜单",1,0));
                // 二级菜单
                MENU_LIST.add(new Menu(6,"1-2菜单",2,1));
                        // 三级菜单
                        MENU_LIST.add(new Menu(7,"1-2-1菜单",1,6));
                // 二级菜单
                MENU_LIST.add(new Menu(8,"1-3菜单",3,1));
                MENU_LIST.add(new Menu(5,"1-1菜单",1,1));

        MENU_LIST.add(new Menu(2,"2菜单",2,0));
                MENU_LIST.add(new Menu(7,"2-1菜单",1,2));
                MENU_LIST.add(new Menu(9,"2-2菜单-4",2,2));

        MENU_LIST.add(new Menu(3,"3菜单",3,0));
        MENU_LIST.add(new Menu(4,"4菜单",4,0));

    }
}

3. 通过 MenuList 构建目录树

public class DemoTest {

    private static final List<Menu> MENU_LIST=MenuList.MENU_LIST;

    public static void main(String[] args) throws JsonProcessingException {
    	// 获取所有的根目录
        List<Menu> rootMenus = MENU_LIST.stream().filter(item -> item.getPid().equals(0)).collect(Collectors.toList());
        // 拿着每一个根目录去构建它的子树
        // 孤单的根目录 --> 一个成熟的树
        List<Menu> result = rootMenus.stream().map(item -> buildChildrenTree(item)).collect(Collectors.toList());
        // 转为 json数据
        System.out.println(new ObjectMapper().writeValueAsString(result));
    }

    private static Menu buildChildrenTree(Menu rootMenu){
        List<Menu> children = MENU_LIST.stream().filter(item -> item.getPid().equals(rootMenu.getId())).collect(Collectors.toList());
        // 如果没有子菜单，就返回自己
        if (ObjectUtils.isEmpty(children)){
            return rootMenu;
        }
        // 有子菜单 就会创建一个集合，将集合作为自己的children，添加进去
        List<Menu> childrenResult=new ArrayList<>();
        children.forEach(item->{
            // 递归，构建 每一个子菜单的children
            childrenResult.add(buildChildrenTree(item));
        });
        // 通过 orderNum 进行排序
        childrenResult.sort(Comparator.comparingInt(item -> item.getOrderNum()));
        rootMenu.setChildren(childrenResult);
        return rootMenu;
    }
}

4. 最终结果树

[{
	"id": 1,
	"label": "1菜单",
	"orderNum": 1,
	"pid": 0,
	"children": [{
		"id": 5,
		"label": "1-1菜单",
		"orderNum": 1,
		"pid": 1,
		"children": null
	}, {
		"id": 6,
		"label": "1-2菜单",
		"orderNum": 2,
		"pid": 1,
		"children": [{
			"id": 7,
			"label": "1-2-1菜单",
			"orderNum": 1,
			"pid": 6,
			"children": null
		}]
	}, {
		"id": 8,
		"label": "1-3菜单",
		"orderNum": 3,
		"pid": 1,
		"children": null
	}]
}, {
	"id": 2,
	"label": "2菜单",
	"orderNum": 2,
	"pid": 0,
	"children": [{
		"id": 7,
		"label": "2-1菜单",
		"orderNum": 1,
		"pid": 2,
		"children": null
	}, {
		"id": 9,
		"label": "2-2菜单-4",
		"orderNum": 2,
		"pid": 2,
		"children": null
	}]
}, {
	"id": 3,
	"label": "3菜单",
	"orderNum": 3,
	"pid": 0,
	"children": null
}, {
	"id": 4,
	"label": "4菜单",
	"orderNum": 4,
	"pid": 0,
	"children": null
}]