代碼是這樣的：packagemainimport("fmt""io""net/http""log")//獲取大小的借口typeSizerinterface{Size()int64}//helloworld,thewebserverfuncHelloServer(whttp.ResponseWriter,r*http.Request){if"POST"==r.Method{file,_,err:=r.FormFile("userfile")iferr!=nil{http.Error(w,err.Error(),500)return}fmt.Fprintf(w,"上傳文件的大小為:%d",file.(Sizer).Size())return}//上傳頁(yè)面w.Header().Add("Content-Type","text/html")w.WriteHeader(200)html:=`Sendthisfile:`io.WriteString(w,html)}funcmain(){http.HandleFunc("/hello",HelloServer)err:=http.ListenAndServe(":12345",nil)iferr!=nil{log.Fatal("ListenAndServe:",err)}}現(xiàn)在問(wèn)題在于這一句：file.(Sizer).Size()。Sizer的interface明顯是程序里定義的，沒有任何實(shí)現(xiàn)接口的代碼，但是這個(gè)語(yǔ)句是怎么工作的？