java 一种树的写法

 左侧菜单树的写法　　

一 for循环添加

    public List<MenuTreeVo> list(String pid, String menuGroup) {
        List<MenuTreeVo> pList=new ArrayList();
        Map<String, MenuTreeVo>map=new HashMap<>();
        List<MenuTreeVo> list=menuTreeMapper.list(pid,menuGroup);
        for(MenuTreeVo menuTreeVo:list){
            if(StringUtils.isEmpty(menuTreeVo.getPid())||"0".equals(menuTreeVo.getPid())){
                pList.add(menuTreeVo);
            }
            map.put(menuTreeVo.getId(),menuTreeVo);
        }
        for(MenuTreeVo menuTreeVo:list){
            if(!StringUtils.isEmpty(menuTreeVo.getPid())){
                MenuTreeVo pMenuTreeVo=map.get(menuTreeVo.getPid());
                if(pMenuTreeVo!=null){
                    if(pMenuTreeVo.getChildrens()==null){
                        pMenuTreeVo.setChildrens(new ArrayList<>());
                    }
                    pMenuTreeVo.getChildrens().add(menuTreeVo);
                }

            }
        }

        return pList;
    }

　　第一次for循环将父级元素存到list   并将所有元素存到map

　　第二次for循环将map中的元素根据pid存放到父级元素中

二 mybatis中sql语句

<select id="list2" resultMap="MenuTreeVotMap2">
    SELECT
    id,
    pid,
    name,
    url,
    menu_key,
    order_num,
    date_format(create_time,'%Y-%m-%d %H:%i:%s') as create_time,
    date_format(update_time,'%Y-%m-%d %H:%i:%s') as date_format,
    account,
    LEVEL,
    menu_group,
    component
    FROM
    sys_menu_tree_table
    <where>
    <if test="pid==null and ''==pid">
      and pid = '0'
    </if>
      <if test="null!=pid and ''!= pid">
        and pid=#{pid}
      </if>
      <if test="null!=menuGroup and ''!= menuGroup">
        and menu_group=#{menuGroup}
      </if>
    </where>
    order by order_num
  </select>

<resultMap id="MenuTreeVotMap2" type="com.zqny.core.module.common.vo.MenuTreeVo">
    <id column="id" jdbcType="VARCHAR" property="id" />
    <result column="pid" jdbcType="VARCHAR" property="pid" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <result column="url" jdbcType="VARCHAR" property="url" />
    <result column="menu_key" jdbcType="VARCHAR" property="menuKey" />
    <result column="order_num" jdbcType="INTEGER" property="orderNum" />
    <result column="create_time" jdbcType="TIMESTAMP" property="createTime" />
    <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" />
    <result column="account" jdbcType="VARCHAR" property="account" />
    <result column="level" jdbcType="INTEGER" property="level" />
    <result column="menu_group" jdbcType="VARCHAR" property="menuGroup" />
    <result column="component" jdbcType="VARCHAR" property="component" />
    <collection property="childrens" ofType="MenuTreeVotMap2"  javaType="java.util.List"
                select="com.zqny.core.mapper.MenuTreeMapper.list2" column="{pid=id,menuGroup=menu_group}">
    </collection>
  </resultMap>

通过collection来继续调用sql语句达到树状分支的目的