Java-根据父级id将List结构转Tree结构

1. 方法一: List的stream()方法

public ResultData queryMenuList() {
　　 // 获取所有数据List
    List<MenuVo> list = MenuDao.queryMenuList();
　　 // 通过list.stream()将List结构转成Tree结构并返回
    List<MenuVo> collection = list.stream()
            .filter(o -> StrUtil.equals("-1", o.getPIdString()))
            .peek(o -> o.setChildList(getChildList(o, list)))
            .collect(Collectors.toList());
    return ResultData.success(Constants.QUERY_SUCCESS, collection, collection.size());
}
// 递归方法
private List<MenuVo> getChildList(MenuVo MenuVo, List<MenuVo> list) {
    return list.stream()
            .filter(o -> StrUtil.equals(String.valueOf(MenuVo.getIdString()), o.getPIdString()))
            .peek(o -> o.setChildList(getChildList(o, list)))
            .collect(Collectors.toList());
}

2. 方法二: mybatis映射

//实体类
@Data
public class Menu{
  // id
  private int id;
  // 菜单名称
  private String name;
  // 上级菜单id
  private int parentId;
  // 子集菜单列表
  private List<Menu> childList;
  
} 

// XML
<resultMap id="MenuMap" type="com.demo.study.entity.Menu">
    <result column="id" property="id"/>
    <result column="name" property="name"/>
    <result column="parent_id" property="parentId"/>
    <collection column="id" property="childList"  select="queryMenuList"/>
</resultMap>

  <select id="queryMenuList" resultMap="MenuMap">
      select 
        *
      from 
        t_menu
      where 
        parent_id = #{id}
  </select>