有关map[string]interface{}用法求教

代码如下，想问下最后一个语句怎么理解？ ``` type Domain_list_t map[string]interface{} var DomainList Domain_list_t //DomainList.txt 文件存放域名列表，一行一个 func LoadOrCreateDomainList() bool { buf, err := ioutil.ReadFile("./Domainlist.txt") if err != nil { return false } DomainList = make(Domain_list_t) //domain为Domainlist.txt里的一行，假设是第一行是 www.baidu.com for _, domain := range strings.Split(string(buf), "\n") { //subs为domain的各级域名组成的切片 {www,baidu,com} subs := strings.Split(strings.Trim(domain, "\r "), ".") if len(subs) == 0 || len(domain) == 0 || domain[0] == '#' { continue } top := DomainList //i=2时 for i := len(subs) - 1; i >= 0; i-- { //sub[2]="com"，top["com"]==nil if top[subs[i]] == nil { //初始化top["com"] top[subs[i]] = make(Domain_list_t) } if i == 0 { top[subs[0]].(Domain_list_t)["_"] = true } //这里不明白，为啥 top["com"] 后面还能加点号，为啥 Domain_list_t 要加括号 top = top[subs[i]].(Domain_list_t) } } ```