Java集合转成树的三种方法（listToTree）

说明：

以stream的方式进行转化
转成json处理再转成list

实体：

package com.utils;

import java.util.List;

/**
 * @Description:
 * @Author: 
 * @Date: 2021/12/16 9:35
 */
public class TestEntity {
    private String name;
    private String code;
    private String parentCode;
    private Integer age;
    private List<TestEntity> children;

    public List<TestEntity> getChildren() {
        return children;
    }

    public void setChildren(List<TestEntity> children) {
        this.children = children;
    }
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    public String getParentCode() {
        return parentCode;
    }

    public void setParentCode(String parentCode) {
        this.parentCode = parentCode;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "TestEntity{" +
                "name='" + name + '\'' +
                ", code='" + code + '\'' +
                ", parentCode='" + parentCode + '\'' +
                ", age=" + age +
                ", children=" + children +
                '}';
    }
}

方法一：

public static void main(String[] args) {
    List<TestEntity> list1 = new ArrayList<>();
    TestEntity testEntity = new TestEntity();
    testEntity.setName("张三");
    testEntity.setCode("45612");
    testEntity.setParentCode("");
    testEntity.setAge(0);
    TestEntity testEntity1 = new TestEntity();
    testEntity1.setName("李四");
    testEntity1.setCode("342");
    testEntity1.setParentCode("45612");
    testEntity1.setAge(0);
    TestEntity testEntity2 = new TestEntity();
    testEntity2.setName("王五");
    testEntity2.setCode("435342");
    testEntity2.setParentCode("342");
    testEntity2.setAge(0);
    TestEntity testEntity3 = new TestEntity();
    testEntity3.setName("王6");
    testEntity3.setCode("11");
    testEntity3.setParentCode("");
    testEntity3.setAge(0);
    list1.add(testEntity);
    list1.add(testEntity1);
    list1.add(testEntity2);
    list1.add(testEntity3);
    
    //要点
    Map<String, List<TestEntity>> parentMap = list1.stream().collect(Collectors.groupingBy(TestEntity::getParentCode));
    list1.forEach(item -> { item.setChildren(parentMap.get(item.getCode())); });
    //过滤出根节点集合,根节点已经包含了孩子节点
    String root = "";
    List<TestEntity> testEntities = list1.stream().filter(item -> root.equals(item.getParentCode())).collect(Collectors.toList());

方法二:

这个需要引入json包

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.74</version>
</dependency>

    /**
     * @param testEntities  :要转成树的集合
     * @param keyMark       :集合中实体父子关系字段的名称
     * @param parentKeyMark :集合中实体父子关系字段的名称
     * @param childMark     :集合中实体孩子集合字段的名称
     * @return List:树集合
     * @Date : 2021/12/16 15:07
     * @description :testEntities、testEntities、testEntities三个就是字段字符串，如TestEntity中有字段String id，String parentId，String name，List<TestEntity> child字段，方法的
     * keyMark、parentKeyMark、childMark就是“id”、"parentId"、"child"
     */
    public static List<TestEntity> listToTree(List<TestEntity> testEntities, String keyMark, String parentKeyMark, String childMark) {
        JSONArray jsonArray = JSONArray.parseArray(JSONObject.toJSONString(testEntities));
        JSONArray jsonArrayTree = new JSONArray();
        JSONObject hash = new JSONObject();
        //将数组转为Object的形式，key为数组中的id
        for (int i = 0; i < jsonArray.size(); i++) {
            JSONObject json = (JSONObject) jsonArray.get(i);
            hash.put(json.getString(keyMark), json);
        }
        //遍历结果集
        for (int j = 0; j < jsonArray.size(); j++) {
            //单条记录
            JSONObject aVal = (JSONObject) jsonArray.get(j);
            //在hash中取出key为单条记录中pid的值
            if (null != aVal.get(parentKeyMark) && !"".equals(aVal.get(parentKeyMark))) {
                JSONObject hashVP = (JSONObject) hash.get(aVal.get(parentKeyMark).toString());
                //如果记录的pid存在，则说明它有父节点，将她添加到孩子节点的集合中
                if (hashVP != null) {
                    //检查是否有child属性
                    if (hashVP.get(childMark) != null) {
                        JSONArray ch = (JSONArray) hashVP.get(childMark);
                        ch.add(aVal);
                        hashVP.put(childMark, ch);
                    } else {
                        JSONArray ch = new JSONArray();
                        ch.add(aVal);
                        hashVP.put(childMark, ch);
                    }
                } else {
                    jsonArrayTree.add(aVal);
                }
            } else {
                jsonArrayTree.add(aVal);
            }

        }
        return jsonArrayTree.toJavaList(TestEntity.class);
    }

方法三:

/**
 * 使用递归方法建树
 * @param testEntity 子节点集合
 * @return List<TestEntity>
 */
public static List<TestEntity> buildTree(List<TestEntity> testEntity) {
   List<TestEntity> trees = new ArrayList<>();
   for (TestEntity TestEntity : testEntity) {
      String parentCode = TestEntity.getParentCode();
      if (null == parentCode || "-".equals(parentCode)) {
         trees.add(findChildren(TestEntity, testEntity));
      }
   }
   return trees;
}

/**
 * 递归查找子节点
 * @param TestEntity   菜单数对象
 * @param testEntity 子节点
 * @return MenuTree
 */
private static TestEntity findChildren(TestEntity TestEntity, List<TestEntity> testEntity) {
   for (TestEntity it : testEntity) {
      if (TestEntity.getCode().equals(it.getParentCode())) {
         if (TestEntity.getChildren() == null) {
            TestEntity.setChildren(new ArrayList<TestEntity>());
         }
         TestEntity.getChildren().add(findChildren(it, testEntity));
      }
   }
   return TestEntity;
}