final or val function parameter or in Kotlin?

Question

Why has Kotlin removed the final or val function parameter which is very useful in Java?

fun say(val msg: String = "Hello World") {
    msg = "Hello To Me" // would give an error here since msg is val    
                        //or final
    ...
    ...
    ...
}

It's nice to wish for something and then find out that it is already there, isn't it? :) — voddan, Nov 12 '16 at 17:07
I am using kotlin version 1.3.31 and its not allowing me to take var as a function parameter — B.shruti, Jul 04 '19 at 05:48

score 79 · Accepted Answer · answered Nov 12 '16 at 13:43

79

Kotlin function parameters are final. There is no val or final keyword because that's the default (and can't be changed).

answered Nov 12 '16 at 13:43

JB Nizet

678,734
91
1,224
1,255

2

Related 3 year old discussion here: https://discuss.kotlinlang.org/t/function-parameters-are-val-not-var/897 – tim_yates Nov 12 '16 at 13:45
Yeah I did not even noticed. – LEMUEL ADANE Nov 12 '16 at 16:34
Is it possible though to set a method as final, so that if any class extends from the current class, it won't be able to override this function? – android developer Oct 29 '17 at 11:53
@androiddeveloper that's the default. Read https://kotlinlang.org/docs/reference/classes.html#overriding-methods. And while you're at it, read the rest of the manual, which is short, and easy to read. – JB Nizet Oct 29 '17 at 11:55
@JBNizet Weird thing is that in my case it's not for an abstract class I've created. It allows me to override all functions. – android developer Oct 29 '17 at 14:03
@androiddeveloper Ask your own question, containing all the relevant code. – JB Nizet Oct 29 '17 at 14:07

score 11 · Answer 2 · edited May 22 '23 at 17:53

After Kotlin M5.1 support of mutable parameters has been removed. In earlier versions that can be achieved using

fun foo(var x: Int) {
  x = 5
}

According to Kotlin developers, the main reasons for removing this feature are

It was confusing. People tend to think that this means passing a parameter by reference, which we do not support. It is costly at runtime.
Another source of confusion is primary constructors: val or var in a constructor declaration means something different from the same thing in a function declaration. Namely, it creates a property.
Also, mutating parameters is not good style, so writing val or var in front of a parameter in a function, catch block or for-loop is no longer allowed.

Summary - All parameter values are val now. You have to introduce separate variable for re-initialising. Example:

fun say(val msg: String) {
    var tempMsg = msg
    if(yourConditionSatisfy) {
       tempMsg = "Hello To Me"
    }
}

@DanielWilson I am not sure what guidelines you follow. But generally I append temp in front variable name. Or better you can suggest edit. — Rahul, Jan 08 '19 at 18:16

score 0 · Answer 3 · answered Nov 05 '20 at 14:29

And another reason is that val and var differ by only one letter. This can be very confusing. So for function parameters they removed the option completely. Thus eliminating the confusion in this one small area (yet keeping it everywhere else--go figure).

score 0 · Answer 4 · answered Jul 24 '21 at 18:38

0

This decision was made to avoid fragile base class problem. It happens when a small change in base classes (superclasses) makes subclasses malfunction.

answered Jul 24 '21 at 18:38

Nazanin Nasab

595
8
18

final or val function parameter or in Kotlin?

4 Answers4

Linked