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這篇文章主要介紹“Java遞歸實現樹形結構的方式有哪些”，在日常操作中，相信很多人在Java遞歸實現樹形結構的方式有哪些問題上存在疑惑，小編查閱了各式資料，整理出簡單好用的操作方法，希望對大家解答”Java遞歸實現樹形結構的方式有哪些”的疑惑有所幫助！接下來，請跟著小編一起來學習吧！
在開發的過程中，很多業務場景需要一個樹形結構的結果集進行前端展示，也可以理解為是一個無限父子結構，常見的有報表指標結構、菜單結構等。Java中遞歸實現樹形結構的兩種常見方式如下：
Java7及以下純Java遞歸實現
Java8及以上借助lamda表達式實現
Java實體類NodePO對應數據庫表
package com.wbs.pojo;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.util.List;
@Data
@NoArgsConstructor
public class NodePO {
/**
* 當前節點id
*/
private String id;
/**
* 當前節點名稱
*/
private String name;
/**
* 父級節點id
*/
private String parentId;
/**
* 當前節點序號
*/
private String orderNo;
/**
* 子集節點
*/
private List<NodePO> children;
/**
* 構造函數
* @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;
}
}自己造一些數據模擬從數據庫中查詢出來的數據：
static final List<NodePO> nodePOs = Arrays.asList(
new NodePO("1","一級節點1",null,"_0001"),
new NodePO("2","二級節點1.1","1","_0002"),
new NodePO("3","二級節點1.2","1","_0003"),
new NodePO("4","一級節點2",null,"_0004"),
new NodePO("5","二級節點2.1","4","_0005"),
new NodePO("6","二級節點2.2","4","_0006"),
new NodePO("7","三級節點2.2.1","6","_0007"),
new NodePO("8","一級節點3",null,"_0008"),
new NodePO("9","二級節點3.1","8","_0009"),
new NodePO("10","三級節點3.1.1","9","_0010"),
new NodePO("11","四級節點3.1.1.1","10","_0011"),
new NodePO("12","五級節點3.1.1.1.1","11","_0012")
);從開發的過程中發現直接操作實體類集合，專門指定某一個實體類封裝的方法是不具有普適性的，所以將實體類集合統一轉化為Map集合，操作方便，具有一定的普適性：
List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(jsonObject);
BeanMapUtils自己簡單封裝一個工具類（不懼普適性勿噴）：
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實體類和map相互轉換工具類
*/
public class BeanMapUtils {
/**
* 將實體類對象屬性轉化為map對象
* @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;
}
/**
* 將map對象中轉化為實體類對象
* @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;
}
/**
* 通過lambda表達式將List<JavaBean>轉化為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());
}
/**
* 通過lambda表達式將List<Map<String, Object>>轉化為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依賴下的工具包，spring-core核心依賴只要導入spring-boot-starter依賴即可
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter</artifactId> <version>2.2.0.RELEASE</version> </dependency>
既然是Java7及以下實現方式，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;
}
}
//如果沒有發生位置互換，則退出循環
if (!flag){
break;
}
}
return list;
}給定一個節點，獲取它的所有子節點：
/**
* Java7及以下版本獲取子節點的方式
* @param parentNode
* @param allList
* @return
*/
public static List<Map<String, Object>> getJava7Children(Map<String,Object> parentNode,List<Map<String, Object>> allList){
//存放當前節點的直系子節點
List<Map<String, Object>> curNodeChildrenList = Lists.newArrayList();
//存放直系子節點以外的節點
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;
}
//每一層級都進行排序
curNodeChildrenList = sortJava7Map(curNodeChildrenList);
//迭代直系子節點再獲取子節點
for (Map<String, Object> map : curNodeChildrenList) {
map.put("children",getJava7Children(map,otherNodeList));
}
return curNodeChildrenList;
}給出一個結果集，構建樹形結果集：
/**
* 使用Java7的方式獲取樹形結構
* @param allList
* @return
*/
public static List<Map<String, Object>> getJava7ResultTree(List<Map<String, Object>> allList){
//存放所有的一級節點
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);
}獲取樹形結構：
//轉化為Map集合 List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(nodePOs); //獲取樹形結構 List<Map<String, Object>> java7ResultTree = getJava7ResultTree(mapList); //打印輸出 System.out.println(JSON.toJSONString(java7ResultTree));
打印結果：
[{"orderNo":"_0001","children":[{"orderNo":"_0002","children":[],"name":"二級節點1.1","id":"2","parentId":"1"},{"orderNo":"_0003","children":[],"name":"二級節點1.2","id":"3","parentId":"1"}],"name":"一級節點1","id":"1"},{"orderNo":"_0004","children":[{"orderNo":"_0005","children":[],"name":"二級節點2.1","id":"5","parentId":"4"},{"orderNo":"_0006","children":[{"orderNo":"_0007","children":[],"name":"三級節點2.2.1","id":"7","parentId":"6"}],"name":"二級節點2.2","id":"6","parentId":"4"}],"name":"一級節點2","id":"4"},{"orderNo":"_0008","children":[{"orderNo":"_0009","children":[{"orderNo":"_0010","children":[{"orderNo":"_0011","children":[{"orderNo":"_0012","children":[],"name":"五級節點3.1.1.1.1","id":"12","parentId":"11"}],"name":"四級節點3.1.1.1","id":"11","parentId":"10"}],"name":"三級節點3.1.1","id":"10","parentId":"9"}],"name":"二級節點3.1","id":"9","parentId":"8"}],"name":"一級節點3","id":"8"}]
樹形結構搞定！
Java7的方式雖然實現了樹形結構，但是有一定的缺點，比如：代碼量比較大，邏輯相對較復雜，那Java8是如何簡化，如下所示：
既然Java8有lamda表達式，那代碼我們能省就省，先看排序，一行代碼搞定：
/**
* 根據orderNo排序樹形結構的每一個層級
* @param list
* @return
*/
public static List<Map<String, Object>> sortJava8Map(List<Map<String, Object>> list){
if(CollectionUtils.isEmpty(list)){
return Lists.newArrayList();
}
//關鍵之處，一行代碼搞定
list.sort(Comparator.comparing(m -> String.valueOf(m.get("orderNo"))));
return list;
}給定一個節點，獲取它的所有子節點：
釋義：
filter： 過濾，相當于for循環，再if條件判斷。
peek： 給定一個節點，往它的children塞子節點。
/**
* 根據父級節點獲取所有的子集節點
* @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());
}給出一個結果集，構建樹形結果集：
/**
* 獲取樹形結構
* @param mapList
* @return treeList 樹形結果集
*/
public static List<Map<String, Object>> getJava8ResultTree(List<Map<String, Object>> mapList){
if (CollectionUtils.isEmpty(mapList)){
return Lists.newArrayList();
}
//filter過濾出所有的一級節點
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());
}獲取樹形結構：
//轉化為Map集合 List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(nodePOs); //獲取樹形結構 List<Map<String, Object>> java8ResultTree = getJava8ResultTree(mapList); //打印輸出 System.out.println(JSON.toJSONString(java8ResultTree));
打印結果：
[{"orderNo":"_0001","children":[{"orderNo":"_0002","children":[],"name":"二級節點1.1","id":"2","parentId":"1"},{"orderNo":"_0003","children":[],"name":"二級節點1.2","id":"3","parentId":"1"}],"name":"一級節點1","id":"1"},{"orderNo":"_0004","children":[{"orderNo":"_0005","children":[],"name":"二級節點2.1","id":"5","parentId":"4"},{"orderNo":"_0006","children":[{"orderNo":"_0007","children":[],"name":"三級節點2.2.1","id":"7","parentId":"6"}],"name":"二級節點2.2","id":"6","parentId":"4"}],"name":"一級節點2","id":"4"},{"orderNo":"_0008","children":[{"orderNo":"_0009","children":[{"orderNo":"_0010","children":[{"orderNo":"_0011","children":[{"orderNo":"_0012","children":[],"name":"五級節點3.1.1.1.1","id":"12","parentId":"11"}],"name":"四級節點3.1.1.1","id":"11","parentId":"10"}],"name":"三級節點3.1.1","id":"10","parentId":"9"}],"name":"二級節點3.1","id":"9","parentId":"8"}],"name":"一級節點3","id":"8"}]
樹形結構搞定！兩種實現方式對比一下，你就說Java8的方式哇塞不哇塞！！！
到此，關于“Java遞歸實現樹形結構的方式有哪些”的學習就結束了，希望能夠解決大家的疑惑。理論與實踐的搭配能更好的幫助大家學習，快去試試吧！若想繼續學習更多相關知識，請繼續關注億速云網站，小編會繼續努力為大家帶來更多實用的文章！
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