java组装树形结构List问题

[
  {
    "id": 1, 
    "parentId": 0, 
    "userName": "AA", 
    "children": [
      {
        "id": 11, 
        "parentId": 1, 
        "userName": "AA-AA", 
        "children": [
          {
            "id": 111, 
            "parentId": 11, 
            "userName": "AA-AA-AA", 
            "children": []
          }
        ]
      }
    ]
  },
  {
    "id": 2, 
    "parentId": 0, 
    "userName": "BB", 
    "children": [
      {
        "id": 22, 
        "parentId": 2, 
        "userName": "BB-BB", 
        "children": []
      }
    ]
  }
]

import java.util.List;
public class UserTree {
    private Integer id;
    private Integer parentId;
    private String userName;
    private List<UserTree> children;
    public UserTree(Integer id, Integer parentId, String userName) {
        this.id = id;
        this.parentId = parentId;
        this.userName = userName;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public Integer getParentId() {
        return parentId;
    }
    public void setParentId(Integer parentId) {
        this.parentId = parentId;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public List<UserTree> getChildren() {
        return children;
    }
    public void setChildren(List<UserTree> children) {
        this.children = children;
    }
    @Override
    public String toString() {
        return "UserTree{" +
                "id=" + id +
                ", parentId=" + parentId +
                ", userName='" + userName + '\'' +
                ", children=" + children.toString() +
                '}';
    }
}

import java.util.ArrayList;
import java.util.List;
/**
 * 测试用户树形结构
 * @author 测试
 */
public class TestUserTree {
    public static void main(String[] args) {
        // 模拟从数据库表中查询出的 user数据list
        List<UserTree> userDataList = new ArrayList<>();
        userDataList.add(new UserTree(1, 0, "AA"));
        userDataList.add(new UserTree(11, 1, "AA-AA"));
        userDataList.add(new UserTree(111, 11, "AA-AA-AA"));
        userDataList.add(new UserTree(2, 0, "BB"));
        userDataList.add(new UserTree(22, 2, "BB-BB"));
        // 父级根节点 list
        List<UserTree> rootUserList = new ArrayList<>();
        // 从查询出的数据中 获取 所有父级根节点
        for (UserTree user : userDataList) {
            // parentId为0的是父级
            if (user.getParentId().equals(0)) {
                rootUserList.add(user);
            }
        }
        // 最终的树形结构list
        List<UserTree> userTreeList = new ArrayList<>();
        // 构建树形结构
        for (UserTree rootUser : rootUserList) {
            UserTree user = buildUserTree(userDataList, rootUser);
            userTreeList.add(user);
        }
        System.out.println(userTreeList);
    }
    /**
     * 递归构建树形结构
     * @param userDataList 所有的用户数据list
     * @param userTree 用户对象
     * @return
     */
    public static UserTree buildUserTree(List<UserTree> userDataList, UserTree userTree) {
        List<UserTree> childrenUserList = new ArrayList<>();
        for (UserTree user : userDataList) {
            // 当前数据的 parentId 等于 父节点的 id，则该数据是当前父级节点的子级。
            if (user.getParentId().equals(userTree.getId())) {
                // 递归调用
                childrenUserList.add(buildUserTree(userDataList, user));
            }
        }
        userTree.setChildren(childrenUserList);
        return userTree;
    }
}

@Data
public class TreeNode {
  private String id;
  private String code;
  private String name;
  private String parentId;
  private List<TreeNode> children;
}

  public static List<TreeNode> buildTree(List<Map<String, Object>> oriDataMapList) {
    // 判空
    if (oriDataMapList == null || oriDataMapList.isEmpty()) {
      return new ArrayList<>();
    }
    // 最终要返回的只保留根节点的集合，增删比较多，使用LinkedList
    List<TreeNode> respTreeNodeList = new LinkedList<>();
    // id集合，后面用来判断parentId是否在oriTreeNodeList存在，如果存在则说明是子节点，不存在则认为是根节点
    Set<String> idSet = new HashSet<>(oriDataMapList.size());
    // 拷贝数据
    TreeNode treeNode;
    for (Map map : oriDataMapList) {
      treeNode = new TreeNode();
      treeNode.setId((String) map.get("id"));
      treeNode.setCode((String) map.get("code"));
      treeNode.setName((String) map.get("name"));
      treeNode.setParentId((String) map.get("parentId"));
      // 遍历的时候顺便保存id集合
      idSet.add(treeNode.getId());
      respTreeNodeList.add(treeNode);
    }
    // 获取由parentId作为key的map，同一个父节点的数据已经被汇总成一个list，通过key就能获取
    Map<String, List<TreeNode>> parentIdMap = respTreeNodeList.stream().collect(Collectors.groupingBy(item -> item.getParentId() != null ? item.getParentId() : "nonParent"));
    // 设置children，并从根节点移出不是父节点的数据
    Iterator<TreeNode> iterator = respTreeNodeList.iterator();
    TreeNode node;
    while (iterator.hasNext()) {
      node = iterator.next();
      // 获取当前结点的子节点集合
      node.setChildren(parentIdMap.get(node.getId()));
      // 如果父id在id集合中存在，则说明他不是根节点，移除
      if (idSet.contains(node.getParentId())) {
        iterator.remove();
      }
    }
    return respTreeNodeList;
  }

  public static void main(String[] args) {
    // 模拟测试数据
    List<Map<String, Object>> mapList = new ArrayList<>();
    Map<String, Object> map;
    for (int i = 0; i < 1000; i++) {
      String parentId = String.valueOf(UUID.randomUUID());
      map = new HashMap<>();
      map.put("id", parentId);
      // %03d 长度为3，不够的补0
      map.put("code", String.format("%03d", i));
      mapList.add(map);
      for (int j = 0; j < 1000; j++) {
        map = new HashMap<>();
        String id = String.valueOf(UUID.randomUUID());
        map.put("id", id);
        map.put("code", String.format("%03d", i) + String.format("%03d", j));
        map.put("parentId", parentId);
        mapList.add(map);
      }
    }
    long start = System.currentTimeMillis();
    List<TreeNode> treeNodeList = buildTree(mapList);
    String msg = String.format("组装【%s】条数据，耗时【%s】ms", mapList.size(), (System.currentTimeMillis() - start));
    System.out.println(msg);
  }