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详解Java递归实现树形结构的两种方式

作者：一宿君　　发布时间：2023-02-18 07:24:47　

标签：Java,递归,树形结构

0、引言

在开发的过程中，很多业务场景需要一个树形结构的结果集进行前端展示，也可以理解为是一个无限父子结构，常见的有报表指标结构、菜单结构等。Java中递归实现树形结构的两种常见方式如下：

Java7及以下纯Java递归实现
Java8及以上借助lamda表达式实现

1、数据准备

Java实体类NodePO对应数据库表

package com.wbs.pojo;

import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

@Data
@NoArgsConstructor
public class NodePO {

/**
* 当前节点id
*/
private String id;

/**
* 当前节点名称
*/
private String name;

/**
* 父级节点id
*/
private String parentId;

/**
* 当前节点序号
*/
private String orderNo;

/**
* 子集节点
*/
private List<NodePO> children;

/**
* 构造函数
* @param id
* @param name
* @param parentId
* @param orderNo
*/
public NodePO(String id,String name,String parentId,String orderNo){
this.id = id;
this.name = name;
this.parentId = parentId;
this.orderNo = orderNo;
}
}

自己造一些数据模拟从数据库中查询出来的数据：

static final List<NodePO> nodePOs = Arrays.asList(
new NodePO("1","一级节点1",null,"_0001"),
new NodePO("2","二级节点1.1","1","_0002"),
new NodePO("3","二级节点1.2","1","_0003"),

new NodePO("4","一级节点2",null,"_0004"),
new NodePO("5","二级节点2.1","4","_0005"),
new NodePO("6","二级节点2.2","4","_0006"),
new NodePO("7"," * 节点2.2.1","6","_0007"),

new NodePO("8","一级节点3",null,"_0008"),
new NodePO("9","二级节点3.1","8","_0009"),
new NodePO("10"," * 节点3.1.1","9","_0010"),
new NodePO("11","四级节点3.1.1.1","10","_0011"),
new NodePO("12","五级节点3.1.1.1.1","11","_0012")
);

2、类型转化

从开发的过程中发现直接操作实体类集合，专门指定某一个实体类封装的方法是不具有普适性的，所以将实体类集合统一转化为Map集合，操作方便，具有一定的普适性：

List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(jsonObject);

BeanMapUtils自己简单封装一个工具类（不惧普适性勿喷）：

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());
}
}

其中org.springframework.cglib.beans.BeanMap;是org.springframework:spring-core依赖下的工具包，spring-core核心依赖只要导入spring-boot-starter依赖即可

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

3、递归实现方法

3.1、Java7及以下纯Java递归实现

既然是Java7及以下实现方式，那排序也用最原始的冒泡排序：

/**
* 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;
}

给定一个节点，获取它的所有子节点：

/**
* 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;
}

给出一个结果集，构建树形结果集：

/**
* 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);
}

获取树形结构：

//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));

打印结果：

[{"orderNo":"_0001","children":[{"orderNo":"_0002","children":[],"name":"二级节点1.1","id":"2","parentId":"1"},{"orderNo":"_0003","children":[],"name":"二级节点1.2","id":"3","parentId":"1"}],"name":"一级节点1","id":"1"},{"orderNo":"_0004","children":[{"orderNo":"_0005","children":[],"name":"二级节点2.1","id":"5","parentId":"4"},{"orderNo":"_0006","children":[{"orderNo":"_0007","children":[],"name":" * 节点2.2.1","id":"7","parentId":"6"}],"name":"二级节点2.2","id":"6","parentId":"4"}],"name":"一级节点2","id":"4"},{"orderNo":"_0008","children":[{"orderNo":"_0009","children":[{"orderNo":"_0010","children":[{"orderNo":"_0011","children":[{"orderNo":"_0012","children":[],"name":"五级节点3.1.1.1.1","id":"12","parentId":"11"}],"name":"四级节点3.1.1.1","id":"11","parentId":"10"}],"name":" * 节点3.1.1","id":"10","parentId":"9"}],"name":"二级节点3.1","id":"9","parentId":"8"}],"name":"一级节点3","id":"8"}]

树形结构搞定！

3.2、Java8及以上借助lamda表达式实现

Java7的方式虽然实现了树形结构，但是有一定的缺点，比如：代码量比较大，逻辑相对较复杂，那Java8是如何简化，如下所示：

既然Java8有lamda表达式，那代码我们能省就省，先看排序，一行代码搞定：

/**
* 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;
}

给定一个节点，获取它的所有子节点：

释义：
filter： 过滤，相当于for循环，再if条件判断。
peek： 给定一个节点，往它的children塞子节点。

/**
* 根据父级节点获取所有的子集节点
* @param parentNode
* @param allList
* @return
*/
public static List<Map<String, Object>> getJava8Children(Map<String,Object> parentNode, List<Map<String, Object>> allList){
return allList.stream()
.filter(curNode -> ObjectUtils.isNotEmpty(curNode.get("parentId")) && Objects.equals(curNode.get("parentId"),parentNode.get("id")))
.peek(m -> m.put("children", getJava8Children(m,allList))).collect(Collectors.toList());
}

给出一个结果集，构建树形结果集：

/**
* 获取树形结构
* @param mapList
* @return treeList 树形结果集
*/
public static List<Map<String, Object>> getJava8ResultTree(List<Map<String, Object>> mapList){
if (CollectionUtils.isEmpty(mapList)){
return Lists.newArrayList();
}
//filter过滤出所有的一级节点
return mapList.stream().filter(m -> Objects.equals(m.get("parentId"), null) || Objects.equals(m.get("parentId"), ""))
.peek(m -> m.put("children", sortJava8Map(getJava8Children(m, mapList)))).collect(Collectors.toList());
}

获取树形结构：

//转化为Map集合
List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(nodePOs);
//获取树形结构
List<Map<String, Object>> java8ResultTree = getJava8ResultTree(mapList);
//打印输出
System.out.println(JSON.toJSONString(java8ResultTree));