Difference between def and val in Scala

Oftentimes, val and def can get confusing in Scala. Especially when there are no parameters to a function. Let me explain with an example:

object HelloWorld {
    val total: Int = 123

    def totalDef: Int = {
        println("Inside def")
        456
    }

    def main(args: Array[String]) {
        println("Main: "+total)
        println(totalDef)
    }
}

If you notice, the syntax of calling a variable total and a function totalDef is just the same.

This can get confusing and one can easily think that if they are of the same syntax, why not change def to val or vice-versa?

Well, you can't. It's because though the syntax is similar, their compilation and execution is totally different.

A variable is initialized only once at the start of a program whereas a function is initialized every time it is called.

For example:

val test = {
           println("Inside Val")
           20
}
println(test)

When the above code is executed, the output is:

Inside Val
20

This is because when initializing a variable, a block of statements has to be executed. And if you know that in scala, the value of a block is the last line of the block. Hence, while executing the block, Inside Val is printed, and then the value of test i.e. 20 is printed.

No matter if you use test, n number of times, Inside Val will always be printed only once. However, if I make it a function and call it more than once:

def func = {
           println("Inside Def")
           20
}
println(func)
println(func)

The output will be:

Inside Def
20
Inside Def
20

This is because a function is initialized every single time it is called.

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I hope now you understand the difference between val and def in Scala.

Thanks for reading!

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