java生成树形结构

public class System extends BaseEntity  {

   private static final long serialVersionUID = 1L;

   /**
    * 设备名称
    */
   @ApiModelProperty(value = "设备名称")
   private String name;
   /**
    * code
    */
   @ApiModelProperty(value = "code")
   private String code;
   /**
    * true false
    */
   @ApiModelProperty(value = "true false")
   private Boolean collapsed;
   /**
    * 父主键id
    */
   @ApiModelProperty(value = "父主键id")
   private Long parentId;
   @TableField(exist = false)
   private List<System> children;

   private Integer sort; //排序
   @TableField(exist = false) //是否有子集
   private Boolean hasChildren;


}

public R treeNode() {
//1.查询1级菜单
List<System> systemList = systemService.list(new QueryWrapper<System>().eq("parent_id", 0));
List<System> list = systemService.list(); //查询全部
List<System> listNodes = new ArrayList<>();
for (System system : systemList) {
System listNode = findListNode(list, system);
listNodes.add(listNode);
}
return R.data(listNodes);
}

/**
* @param list 所有元素
* @param sys 父节点
* @return 树状jsonNode
*/
private System findListNode(List<System> list, System sys) {
List<System> systemList = new ArrayList<>();
for (System system : list) {
if (sys.getId().equals(system.getParentId())) {
systemList.add(findListNode(list, system));
}
}
sys.setChildren(systemList);
return sys;
}