Call by Value and Call by Name Parameters

While you define Methods with arguments, there are two ways of parameter passing:

Call by Value: Those parameters that are already evaluated when they are passed to a function.

Syntax :    parameter: parameterType

Call by Name : These are the parameters that are not already evaluated but will be evaluated each time a call to function is made.

Syntax : parameter :=> parameterType

Example :

def callByValue(num: Int) = {
  println(s"First fetch: $num. Second fetch: $num")
}

callByValue(scala.util.Random.nextInt)
callByValue(scala.util.Random.nextInt)

def callByName(num: => Int) = {
  println(s"First fetch: $num. Second fetch: $num")
}

callByName(scala.util.Random.nextInt)

callByName(scala.util.Random.nextInt)

Output:

First fetch: 1917657572. Second fetch: 1917657572
First fetch: -1411166275. Second fetch: -1411166275
First fetch: -276172407. Second fetch: 908226387
First fetch: 397353949. Second fetch: 1896525012