Kotlin: val mutableList vs var immutableList. When to use which?

Question

We are encouraged to have immutable variables as much as we can.

But sometimes when I have to modify a list I start to wonder which approach would be better for current situation...

val mutableList = mutableListOf()
// where I can just .add() / .remove() accordingly

or

var immutableList = listOf()
// where I can create a new list (using filter or `+`) each time a change is made

I guess there are different scenarios one is preferred over the other. Hence I would like to know when one should be used over the other

Answer 1

val -> You could think that you can't reassign for the variable.

//that is ok
var a:Int = 1
a=2
//Even you can reassign but you can't change its type
a= "string"  //that's wrong

//that is wrong
val b:Int = 1
b = 2

ListOf -> You could think that you can't insert/delete/alter any element in the list (can't do anything to the content of the list)

var list:List<Int> = listOf(1,2,3,4) //[1,2,3,4]
//you can read list
list.get(0)
list[0]
//but you can't change(/write) the content of the list (insert/delete/alter)
list.set(0, 100)
list.add(5)
list.removeAt(0)

var mutableList:MutableList<Int> = mutableListOf(1,2,3,4) //[1,2,3,4]
//you can read and write
mutableList.get(0)
mutableList.set(0, 100) //[100,2,3,4]
mutableList.add(5)      //[100,2,3,4,5]
mutableList.removeAt(0) //[2,3,4,5]

---

SO combine both of them, you will get four cases

Case 1: var mutableList:MutableList = mutableListOf(1,2,3,4)

//you can reassign 
mutableList = mutableListOf(4,5,6,7) //[4,5,6,7]
//you can alter the content 
mutableList.set(0, 100) //[100,5,6,7]
mutableList.add(8)      //[100,5,6,7,8]
mutableList.removeAt(0) //[5,6,7,8]

Case 2: val mutableList:MutableList = mutableListOf(1,2,3,4)

//you can't reassign 
mutableList = mutableListOf(4,5,6,7) //that's wrong

//you can alter the content 
mutableList.set(0, 100) //[100,2,3,4]
mutableList.add(8)      //[100,2,3,4,8]
mutableList.removeAt(0) //[2,3,4,8]

Case 3: var list:List = ListOf(1,2,3,4)

//you can reassign 
list= ListOf(4,5,6,7) //[4,5,6,7]

//you can't alter the content 
list.set(0, 100) //that's wrong
list.add(8)      //that's wrong
list.removeAt(0) //that's wrong

Case 4: val list:List = ListOf(1,2,3,4)

//you can't reassign 
list= ListOf(4,5,6,7) //that's wrong

//you can't alter the content 
list.set(0, 100) //that's wrong
list.add(8)      //that's wrong
list.removeAt(0) //that's wrong

//the only thing you can do is Read
list.get(0)  //return 1
list[0]      //return 1