go run: cannot run non-main package

here the simple go application. I am getting "go run: cannot run non-main package" error, if I run following code.

package zsdfsdf

import (
    "fmt"
)

func Main() {
    fmt.Println("sddddddd")
}

To fix it, I just need to name the package to main. But I don't understand why I need to do that. I should be able to name the package whatever I want.

Another question, I know main function is the entry point of the program, you need it. otherwise it will not work. But I see some codes that didn't have main function still works.

Click on this link, the example at the bottom of the page didn't use package main and main function, and it still works. just curious why.

https://developers.google.com/appengine/docs/go/gettingstarted/usingdatastore

You need to specify in your app.yaml file what your app access point is. Take a look here. You need to specify:

application: zsdfsdf

Also see from that above link:

"Note: When writing a stand-alone Go program we would place this code in package main. The Go App Engine Runtime provides a special main package, so you should put HTTP handler code in a package of your choice (in this case, hello)."

You are correct that all Go programs need the Main method. But it is provided by Google App Engine. That is why your provided example works. Your example would not work locally (not on GAE).