Why does the following Go program need the function's name to be String() and not anything else? -

i'm following book shows following example.

package main  import (     "fmt" )  const (     kb = 1024     mb = 1048576          //kb * 1024     gb = 1073741824       //mb * 1024     tb = 1099511627776    //gb * 1024     pb = 1125899906842624 //tb * 1024 )  type bytesize float64  func (b bytesize) string() string {     switch {     case b >= pb:         return "very big"     case b >= tb:         return fmt.sprintf("%.2ftb", b/tb)     case b >= gb:         return fmt.sprintf("%.2fgb", b/gb)     case b >= mb:         return fmt.sprintf("%.2fmb", b/mb)     case b >= kb:         return fmt.sprintf("%.2fkb", b/kb)     }     return fmt.sprintf("%db", b) }  func main() {     fmt.println(bytesize(2048))     fmt.println(bytesize(3292528.64)) } 

when run program gives me following output (in human readable data size units).

2.00kb 3.14mb 

but when change name of function called string() else, or if lower-case s in string, gives me following output.

2048 3.29252864e+06 

what reason behind that? there string() function attached interface , our bytesize type satisfies interface? mean hell?

when define method named string no parameters returning string, implement stringer interface, documented here: https://golang.org/pkg/fmt/#stringer.