详解Java递归实现树形结构的两种方式

package com.wbs.util;

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import lombok.SneakyThrows;
import org.springframework.cglib.beans.BeanMap;

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

/**
 * @author 一宿君
 * @version Id: BeanMapUtils.java, v 0.1 Administrator Exp $$
 * @date 2022-10-13 14:24:20
 * @desc java实体类和map相互转换工具类
 */
public class BeanMapUtils {

    /**
     * 将实体类对象属性转化为map对象
     * @param t
     * @param <T>
     * @return
     */
    public static <T> Map<String, Object> beanToMap(T t) {
        Map<String, Object> map = new HashMap<>();
        if (t != null) {
            if (t instanceof JSONObject){
                return (JSONObject)t;
            }
            BeanMap beanMap = BeanMap.create(t);
            for (Object key : beanMap.keySet()) {
                map.put(key.toString(), beanMap.get(key));
            }
        }
        return map;
    }


    /**
     * 将map对象中转化为实体类对象
     * @param map
     * @param clazz
     * @param <T>
     * @return
     * @throws Exception
     */
    public static <T> T mapToBean(Map<String, Object> map,Class<T> clazz) throws Exception {
        T bean = clazz.newInstance();
        if (bean instanceof JSONObject){
            JSONObject jsonObject = (JSONObject)bean;
            Set<Map.Entry<String, Object>> entries = map.entrySet();
            for (Map.Entry<String, Object> entry : entries) {
                jsonObject.put(entry.getKey(),entry.getValue());
            }
            return (T)jsonObject;
        }
        BeanMap beanMap = BeanMap.create(bean);
        beanMap.putAll(map);
        return bean;
    }

    /**
     * 通过lambda表达式将List<JavaBean>转化为List<Map<String, Object>>
     * @param objList
     * @param <T>
     * @return
     */
    public static <T> List<Map<String, Object>> listBeanToListMap(List<T> objList) {
        return objList.stream().map(new Function<T, Map<String, Object>>() {
            @Override
            public Map<String, Object> apply(T t) {
                Map<String,Object> map = Maps.newHashMap();
                if (t instanceof JSONObject){
                    return (JSONObject)t;
                }
                BeanMap beanMap = BeanMap.create(t);
                for (Object key : beanMap.keySet()) {
                    map.put(key.toString(), beanMap.get(key));
                }
                return map;
            }
        }).collect(Collectors.toList());
    }

    /**
     * 通过lambda表达式将List<Map<String, Object>>转化为List<JavaBean>
     * @param mapList
     * @param <T>
     * @return
     */
    public static <T> List<T> listMapToListBean(List<Map<String,Object>> mapList,Class<T> clazz) {
        return mapList.stream().map(new Function<Map<String, Object>,T>() {
            @SneakyThrows
            @Override
            public T apply(Map<String, Object> map) {
                T t = clazz.newInstance();
                if (t instanceof JSONObject){
                    return (T)map;
                }
                BeanMap beanMap = BeanMap.create(t);
                beanMap.putAll(map);
                return t;
            }
        }).collect(Collectors.toList());
    }
}

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <version>2.2.0.RELEASE</version>
</dependency>

/**
     * 冒泡排序,小的在前，大的在后
     * @param list
     * @return
     */
    public static List<Map<String, Object>> sortJava7Map(List<Map<String, Object>> list){
        if(CollectionUtils.isEmpty(list)){
            return Lists.newArrayList();
        }
        boolean flag;
        int size = list.size();
        for (int i = 0; i < size - 1; i++) {
            flag = false;
            for (int j = 1; j < size - i; j++) {
                Map<String, Object> frontMap = list.get(j - 1);
                Map<String, Object> afterMap = list.get(j);
                if (String.valueOf(frontMap.get("orderNo")).compareTo(String.valueOf(afterMap.get("orderNo"))) > 0){
                    list.set(j - 1,afterMap);
                    list.set(j,frontMap);
                    flag = true;
                }
            }
            //如果没有发生位置互换，则退出循环
            if (!flag){
                break;
            }
        }
        return list;
    }

/**
     * Java7及以下版本获取子节点的方式
     * @param parentNode
     * @param allList
     * @return
     */
    public static List<Map<String, Object>> getJava7Children(Map<String,Object> parentNode,List<Map<String, Object>> allList){

        //存放当前节点的直系子节点
        List<Map<String, Object>> curNodeChildrenList = Lists.newArrayList();

        //存放直系子节点以外的节点
        List<Map<String, Object>> otherNodeList = Lists.newArrayList();

        Object pId = parentNode.get("id");
        for (Map<String, Object> map : allList) {
            Object curPId = map.get("parentId");
            if (ObjectUtils.isNotEmpty(curPId) && Objects.equals(pId,curPId)){
                curNodeChildrenList.add(map);
            }else {
                otherNodeList.add(map);
            }
        }
        if (curNodeChildrenList.isEmpty()){
            return curNodeChildrenList;
        }
        //每一层级都进行排序
        curNodeChildrenList = sortJava7Map(curNodeChildrenList);

        //迭代直系子节点再获取子节点
        for (Map<String, Object> map : curNodeChildrenList) {
            map.put("children",getJava7Children(map,otherNodeList));
        }
        return curNodeChildrenList;
    }

/**
     * 使用Java7的方式获取树形结构
     * @param allList
     * @return
     */
    public static List<Map<String, Object>> getJava7ResultTree(List<Map<String, Object>> allList){
        //存放所有的一级节点
        List<Map<String, Object>> oneLevelNodeList = Lists.newArrayList();

        for (Map<String, Object> map : allList) {
            if (ObjectUtils.isEmpty(map.get("parentId"))){
                map.put("children",getJava7Children(map,allList));
                oneLevelNodeList.add(map);
            }
        }
        return sortJava8Map(oneLevelNodeList);
    }

//转化为Map集合
List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(nodePOs);
//获取树形结构
List<Map<String, Object>> java7ResultTree = getJava7ResultTree(mapList);
//打印输出
System.out.println(JSON.toJSONString(java7ResultTree));

/**
     * 根据orderNo排序树形结构的每一个层级
     * @param list
     * @return
     */
    public static List<Map<String, Object>> sortJava8Map(List<Map<String, Object>> list){
        if(CollectionUtils.isEmpty(list)){
            return Lists.newArrayList();
        }
        //关键之处，一行代码搞定
        list.sort(Comparator.comparing(m -> String.valueOf(m.get("orderNo"))));
        return list;
    }