詳解Java遞歸實現(xiàn)樹形結(jié)構(gòu)的兩種方式
0、引言
在開發(fā)的過程中,很多業(yè)務(wù)場景需要一個樹形結(jié)構(gòu)的結(jié)果集進行前端展示,也可以理解為是一個無限父子結(jié)構(gòu),常見的有報表指標(biāo)結(jié)構(gòu)、菜單結(jié)構(gòu)等。Java中遞歸實現(xiàn)樹形結(jié)構(gòu)的兩種常見方式如下:
- Java7及以下純Java遞歸實現(xiàn)
- Java8及以上借助lamda表達式實現(xiàn)
1、數(shù)據(jù)準(zhǔn)備
Java實體類NodePO對應(yīng)數(shù)據(jù)庫表
package com.wbs.pojo; import lombok.Data; import lombok.NoArgsConstructor; import java.util.List; @Data @NoArgsConstructor public class NodePO { /** * 當(dāng)前節(jié)點id */ private String id; /** * 當(dāng)前節(jié)點名稱 */ private String name; /** * 父級節(jié)點id */ private String parentId; /** * 當(dāng)前節(jié)點序號 */ private String orderNo; /** * 子集節(jié)點 */ private List<NodePO> children; /** * 構(gòu)造函數(shù) * @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(shù)據(jù)模擬從數(shù)據(jù)庫中查詢出來的數(shù)據(jù):
static final List<NodePO> nodePOs = Arrays.asList( new NodePO("1","一級節(jié)點1",null,"_0001"), new NodePO("2","二級節(jié)點1.1","1","_0002"), new NodePO("3","二級節(jié)點1.2","1","_0003"), new NodePO("4","一級節(jié)點2",null,"_0004"), new NodePO("5","二級節(jié)點2.1","4","_0005"), new NodePO("6","二級節(jié)點2.2","4","_0006"), new NodePO("7","三級節(jié)點2.2.1","6","_0007"), new NodePO("8","一級節(jié)點3",null,"_0008"), new NodePO("9","二級節(jié)點3.1","8","_0009"), new NodePO("10","三級節(jié)點3.1.1","9","_0010"), new NodePO("11","四級節(jié)點3.1.1.1","10","_0011"), new NodePO("12","五級節(jié)點3.1.1.1.1","11","_0012") );
2、類型轉(zhuǎn)化
從開發(fā)的過程中發(fā)現(xiàn)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(tǒng)一轉(zhuǎn)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(zhuǎn)換工具類 */ public class BeanMapUtils { /** * 將實體類對象屬性轉(zhuǎn)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(zhuǎn)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(zhuǎn)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(zhuǎn)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(dǎo)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>
3、遞歸實現(xiàn)方法
3.1、Java7及以下純Java遞歸實現(xiàn)
既然是Java7及以下實現(xiàn)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(fā)生位置互換,則退出循環(huán) if (!flag){ break; } } return list; }
給定一個節(jié)點,獲取它的所有子節(jié)點:
/** * Java7及以下版本獲取子節(jié)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(dāng)前節(jié)點的直系子節(jié)點 List<Map<String, Object>> curNodeChildrenList = Lists.newArrayList(); //存放直系子節(jié)點以外的節(jié)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(jié)點再獲取子節(jié)點 for (Map<String, Object> map : curNodeChildrenList) { map.put("children",getJava7Children(map,otherNodeList)); } return curNodeChildrenList; }
給出一個結(jié)果集,構(gòu)建樹形結(jié)果集:
/** * 使用Java7的方式獲取樹形結(jié)構(gòu) * @param allList * @return */ public static List<Map<String, Object>> getJava7ResultTree(List<Map<String, Object>> allList){ //存放所有的一級節(jié)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(jié)構(gòu):
//轉(zhuǎn)化為Map集合 List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(nodePOs); //獲取樹形結(jié)構(gòu) List<Map<String, Object>> java7ResultTree = getJava7ResultTree(mapList); //打印輸出 System.out.println(JSON.toJSONString(java7ResultTree));
打印結(jié)果:
[{"orderNo":"_0001","children":[{"orderNo":"_0002","children":[],"name":"二級節(jié)點1.1","id":"2","parentId":"1"},{"orderNo":"_0003","children":[],"name":"二級節(jié)點1.2","id":"3","parentId":"1"}],"name":"一級節(jié)點1","id":"1"},{"orderNo":"_0004","children":[{"orderNo":"_0005","children":[],"name":"二級節(jié)點2.1","id":"5","parentId":"4"},{"orderNo":"_0006","children":[{"orderNo":"_0007","children":[],"name":"三級節(jié)點2.2.1","id":"7","parentId":"6"}],"name":"二級節(jié)點2.2","id":"6","parentId":"4"}],"name":"一級節(jié)點2","id":"4"},{"orderNo":"_0008","children":[{"orderNo":"_0009","children":[{"orderNo":"_0010","children":[{"orderNo":"_0011","children":[{"orderNo":"_0012","children":[],"name":"五級節(jié)點3.1.1.1.1","id":"12","parentId":"11"}],"name":"四級節(jié)點3.1.1.1","id":"11","parentId":"10"}],"name":"三級節(jié)點3.1.1","id":"10","parentId":"9"}],"name":"二級節(jié)點3.1","id":"9","parentId":"8"}],"name":"一級節(jié)點3","id":"8"}]
樹形結(jié)構(gòu)搞定!
3.2、Java8及以上借助lamda表達式實現(xiàn)
Java7的方式雖然實現(xiàn)了樹形結(jié)構(gòu),但是有一定的缺點,比如:代碼量比較大,邏輯相對較復(fù)雜,那Java8是如何簡化,如下所示:
既然Java8有l(wèi)amda表達式,那代碼我們能省就省,先看排序,一行代碼搞定:
/** * 根據(jù)orderNo排序樹形結(jié)構(gòu)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(guān)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(jié)點,獲取它的所有子節(jié)點:
釋義:
filter: 過濾,相當(dāng)于for循環(huán),再if條件判斷。
peek: 給定一個節(jié)點,往它的children塞子節(jié)點。
/** * 根據(jù)父級節(jié)點獲取所有的子集節(jié)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(jié)果集,構(gòu)建樹形結(jié)果集:
/** * 獲取樹形結(jié)構(gòu) * @param mapList * @return treeList 樹形結(jié)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(jié)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(jié)構(gòu):
//轉(zhuǎn)化為Map集合 List<Map<String, Object>> mapList = BeanMapUtils.listBeanToListMap(nodePOs); //獲取樹形結(jié)構(gòu) List<Map<String, Object>> java8ResultTree = getJava8ResultTree(mapList); //打印輸出 System.out.println(JSON.toJSONString(java8ResultTree));
打印結(jié)果:
[{"orderNo":"_0001","children":[{"orderNo":"_0002","children":[],"name":"二級節(jié)點1.1","id":"2","parentId":"1"},{"orderNo":"_0003","children":[],"name":"二級節(jié)點1.2","id":"3","parentId":"1"}],"name":"一級節(jié)點1","id":"1"},{"orderNo":"_0004","children":[{"orderNo":"_0005","children":[],"name":"二級節(jié)點2.1","id":"5","parentId":"4"},{"orderNo":"_0006","children":[{"orderNo":"_0007","children":[],"name":"三級節(jié)點2.2.1","id":"7","parentId":"6"}],"name":"二級節(jié)點2.2","id":"6","parentId":"4"}],"name":"一級節(jié)點2","id":"4"},{"orderNo":"_0008","children":[{"orderNo":"_0009","children":[{"orderNo":"_0010","children":[{"orderNo":"_0011","children":[{"orderNo":"_0012","children":[],"name":"五級節(jié)點3.1.1.1.1","id":"12","parentId":"11"}],"name":"四級節(jié)點3.1.1.1","id":"11","parentId":"10"}],"name":"三級節(jié)點3.1.1","id":"10","parentId":"9"}],"name":"二級節(jié)點3.1","id":"9","parentId":"8"}],"name":"一級節(jié)點3","id":"8"}]
樹形結(jié)構(gòu)搞定!兩種實現(xiàn)方式對比一下,你就說Java8的方式哇塞不哇塞?。?!
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