根据根节点和子节点构造树形结构

package com.cyc.basic.test.list;

import com.alibaba.fastjson.JSON;
import lombok.Data;

import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;

/**
 * @description: 遍历构造树形结构{根节点, 子节点}
 * @version 1.0
 * @author cyc
 * @date 2022/3/11 15:53
 */
public class ParentChildListTest {

    public static void main(String[] args) {
        //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, "父节点1", 1),
                new Menu(3, "子节点1.1", 2),
                new Menu(4, "子节点1.2", 2),
                new Menu(5, "子节点1.3", 2),
                new Menu(6, "父节点2", 1),
                new Menu(7, "子节点2.1", 6),
                new Menu(8, "子节点2.2", 6),
                new Menu(9, "子子节点2.2.1", 7),
                new Menu(10, "子子节点2.2.2", 7),
                new Menu(11, "父节点3", 1),
                new Menu(12, "子节点3.1", 11)
        );

        //获取父节点-方式1
        List<Menu> collect = menus.stream().filter(m -> m.getParentId() == 0).map(
                menu -> {
                    menu.setChildList(getChildrens(menu, menus));
                    return menu;
                }
        ).collect(Collectors.toList());

        //获取父节点-方式2, 使用peek函数, peek 对每个元素执行操作并返回一个新的 Stream
        List<Menu> collect2 = menus.stream().filter(menu -> menu.getParentId() == 0).peek(
                menu -> menu.setChildList(getChildrens(menu, menus))
        ).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 static List<Menu> getChildrens(Menu root, List<Menu> all) {
        List<Menu> children = all.stream().filter(menu -> Objects.equals(menu.getParentId(), root.getId()))
                .map(menu -> {
                            // 此时的menu作为父节点, 继续从所有节点中查询其子节点
                            menu.setChildList(getChildrens(menu, all));
                            return menu;
                        }
                ).collect(Collectors.toList());
        return children;
    }
}


@Data
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;
    }

}

输出结果

[{
	"name": "根节点",
	"childList": [{
		"name": "父节点1",
		"childList": [{
			"name": "子节点1.1",
			"childList": [],
			"id": 3,
			"parentId": 2
		}, {
			"name": "子节点1.2",
			"childList": [],
			"id": 4,
			"parentId": 2
		}, {
			"name": "子节点1.3",
			"childList": [],
			"id": 5,
			"parentId": 2
		}],
		"id": 2,
		"parentId": 1
	}, {
		"name": "父节点2",
		"childList": [{
			"name": "子节点2.1",
			"childList": [{
				"name": "子子节点2.2.1",
				"childList": [],
				"id": 9,
				"parentId": 7
			}, {
				"name": "子子节点2.2.2",
				"childList": [],
				"id": 10,
				"parentId": 7
			}],
			"id": 7,
			"parentId": 6
		}, {
			"name": "子节点2.2",
			"childList": [],
			"id": 8,
			"parentId": 6
		}],
		"id": 6,
		"parentId": 1
	}, {
		"name": "父节点3",
		"childList": [{
			"name": "子节点3.1",
			"childList": [],
			"id": 12,
			"parentId": 11
		}],
		"id": 11,
		"parentId": 1
	}],
	"id": 1,
	"parentId": 0
}]