Overview

Similar to switch  select can also have a default case.  This default case will be executed if no send it or receive operation is  ready on any of the case statement. So in a way default statement prevent the select from blocking forever.  So a very important point to note is that the default statement makes the select non-blocking. If the select statement doesn’t contain a default case then it can block forever until one send or receive operation is ready on any case statement. Let’s see a example to fully understand it

Code

package main

import "fmt"

func main() {
    ch1 := make(chan string)
    select {
    case msg := <-ch1:
        fmt.Println(msg)
    default:
        fmt.Println("Default statement executed")
    }
}

Output

Default statement executed

In the above program have a select statement which is waiting for receive operation on ch1 and a default statement. Since no goroutine is sending to channel ch1, hence the default case is executed and then select exits.

The select statement checks if the value is available in any of the channels in case statements. If it is available  then that case is executed otherwise the default case will be executed immediately. 

Let's see another example where the value is immediately available in one of the channels. We will see that  default case is not executed in this scenario

package main

import "fmt"

func main() {
    ch1 := make(chan string, 1)
    ch1 <- "Some value"
    select {
    case msg := <-ch1:
        fmt.Println(msg)
    default:
        fmt.Println("Default statement executed")
    }
}

Output

Some value

We can see in the above program that the value was available on the ch1 channel that is why that case is executed and the default case is not executed. Same is also evident from the output