为什么对用户定义类型的用户定义String（）的调用会引发“对BitFlag.String的调用中的参数不足”？

I list the code from the book "Programming in Go". I test it but it didn't work well.

error: "not enough arguments in call to BitFlag.String"

Goplayground Code: http://play.golang.org/p/FG23LdS_xK

type BitFlag int

func main() {
    flag := Active | Send
    BitFlag.String();
}

func (flag BitFlag) String() string {
   ...
}

Why do I see this error message?

You need to call String on an instance of BitFlag (here 'flag'), not on the BitFlag type itself.

flag := Active | Send
fmt.Println(strconv.Itoa(int(flag)))
fmt.Println(flag.String())

See this play.golang.org example.

Output:

3
3(Active|Send)