How to realize java recursive processing of authority management menu tree or classification

package com.ieou.capsule.dto.SystemPermissions;
import java.util.List;
/**
 *Function menu class
 */
public class SystemPermissionsTree {
	private String functionCode;
	//Menu code
	private String parentFunctionCode;
	//Parent menu code
	private String functionName;
	//Menu name
	private Boolean flag;
	//True: checked false: not selected
	private List<SystemPermissionsTree> childrenList;
	public String getFunctionCode() {
		return functionCode;
	}
	public void setFunctionCode(String functionCode) {
		this.functionCode = functionCode;
	}
	public String getParentFunctionCode() {
		return parentFunctionCode;
	}
	public void setParentFunctionCode(String parentFunctionCode) {
		this.parentFunctionCode = parentFunctionCode;
	}
	public String getFunctionName() {
		return functionName;
	}
	public void setFunctionName(String functionName) {
		this.functionName = functionName;
	}
	public Boolean getFlag() {
		return flag;
	}
	public void setFlag(Boolean flag) {
		this.flag = flag;
	}
	public List<SystemPermissionsTree> getChildrenList() {
		return childrenList;
	}
	public void setChildrenList(List<SystemPermissionsTree> childrenList) {
		this.childrenList = childrenList;
	}
}

package com.ieou.capsule.util;
import com.ieou.capsule.dto.SystemPermissions.SystemPermissionsTree;
import java.util.ArrayList;
import java.util.List;
public class TreeUtil {
	/**
   *Author: Yi Mufeng Yi
   *Source: CSDN
   *Original text: https://blog.csdn.net/gxgl8811/article/details/72803833
   *Copyright notice: This article is the original article of blogger, please attach the link of blog!
   */
	public static List<SystemPermissionsTree> getTreeList(List<SystemPermissionsTree> entityList) {
		List<SystemPermissionsTree> resultList = new ArrayList<>();
		//Get top level element collection
		String parentCode;
		for (SystemPermissionsTree entity : entityList) {
			parentCode = entity.getParentFunctionCode();
			//The parentcode of the top-level element is null or 0
			if (parentCode == null || "0".equals(parentCode)) {
				resultList.add(entity);
			}
		}
		//Gets the child data collection of each top-level element
		for (SystemPermissionsTree entity : resultList) {
			entity.setChildrenList(getSubList(entity.getFunctionCode(), entityList));
		}
		return resultList;
	}
	/**
   *Get sub data collection
   *
   * @param id
   * @param entityList
   * @return
   * @author jianda
   *@ date 29 may 2017
   */
	private static List<SystemPermissionsTree> getSubList(String id, List<SystemPermissionsTree> entityList) {
		List<SystemPermissionsTree> childList = new ArrayList<>();
		String parentId;
		//Direct subobject of subset
		for (SystemPermissionsTree entity : entityList) {
			parentId = entity.getParentFunctionCode();
			if (id.equals(parentId)) {
				childList.add(entity);
			}
		}
		//Indirect subobjects of subsets
		for (SystemPermissionsTree entity : childList) {
			entity.setChildrenList(getSubList(entity.getFunctionCode(), entityList));
		}
		//Recursive exit condition
		if (childList.size() == 0) {
			return null;
		}
		return childList;
	}
}