如何从数据库中拿取数据来还原一颗指定用户的多叉树

代码如下：

package 算法and数据结构.数据结构.多叉树.文件管理系统.从数据库中拿取数据并建树;

import 算法and数据结构.数据结构.多叉树.文件管理系统.Node.Node;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import static 算法and数据结构.数据结构.多叉树.文件管理系统.连接数据库.getMySQL.getConnection;

public class creat {

    String select = "select file_name from file_system where parent_file_name = '{' and user_name = '{'";

    public creat(Node root){
//        select(root,"893295815@qq.com");

        select(root,"NCEPU_RuanCong");
        System.out.println("=======================前======================");
        text.pre(root);
        System.out.println("=======================中======================");
        text.mid(root);
        System.out.println("=======================后======================");
        text.pos(root);
    }

    private String getSelectSQL(String ...args) {
        StringBuilder stringBuilder = new StringBuilder(args[0]);
        int j = 1;
        for (int i = 0; i < stringBuilder.length(); i++) {
            if (stringBuilder.charAt(i) == '{'){
                stringBuilder.replace(i,i+1,args[j]);
                i=i+args[1].length();
                j++;
            }
        }
        return stringBuilder.toString();
    }

    private void select(Node root,String username){
        if (root == null)return;
        Connection connection=getConnection();
        Statement statement=null;
        ResultSet resultSet=null;
        try {
            statement=getConnection().createStatement();
            System.out.println(getSelectSQL(select,root.date,username));
            resultSet=statement.executeQuery(getSelectSQL(select,root.date,username));
            while (resultSet.next()){
                root.children.add(new Node(resultSet.getString("file_name"),root,1));
            }
            for (int i = 0; i < root.children.size(); i++) {
                select(root.children.get(i),username);
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            if (connection != null){
                try {
                    connection.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
           if (statement != null){
               try {
                   statement.close();
               } catch (SQLException throwables) {
                   throwables.printStackTrace();
               }
           }
            if (resultSet != null) {
                try {
                    resultSet.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }
    }
}
class text{
    public static void main(String[] args) {
//        new creat(new Node("Is 893295815@qq.com Computer",null,1));
        new creat(new Node("Is NCEPU_RuanCong Computer",null,1));

    }
    public static void pre(){

    }
    public static void pre(Node root){
        if(root == null)return;
        System.out.println(root.date);
        for(int i=0 ; i< root.children.size() ;i++){
            pre(root.children.get(i));
        }
    }
    public static void mid(Node root){
        if(root == null)return;
        if (root.children.size() != 0){
            mid(root.children.get(0));
        }
        System.out.println(root.date);
        for(int i = 1 ; i < root.children.size() ; i++){
            mid(root.children.get(i));
        }
    }
    public static void pos(Node root){
        if (root == null){
            return;
        }
        for (int i = 0; i < root.children.size(); i++) {
            pos(root.children.get(i));
        }
        System.out.println(root.date);
    }
}