Hey all, I'm playing around with lists and mutabl...

# getting-started

[Pim]

Hey all,

I'm playing around with lists and mutable lists. I have the following class that I use to test on:

class Persons {
    private val _persons = mutableListOf("Person1", "Person2")
    val entries: List<String>
        get() = _persons.toList()

    fun addPerson(person: String) {
        _persons.add(person)
    }
}

The idea is that the Persons class keeps an internal mutable list. External users of the Persons class can read an immutable version of the list.

Now I tried playing around with casting the received list from entries back to a MutableList<String> to see if I can then start adding things back into the list.
I ran the following code:

fun main() {
    val persons = Persons()
    println(persons.entries.joinToString())
    persons.addPerson("Person3")
    println(persons.entries.joinToString())
    (persons.entries as MutableList<String>).add("Person4")
    println(persons.entries.joinToString())
    (persons.entries as MutableList<String>).also { it.add("Person4") }.joinToString()
    println(persons.entries.joinToString())
}

The output surprises me. I get not warnings or compilation errors, but the "Person4" is never actually added?
Output:

Person1, Person2
Person1, Person2, Person3
Person1, Person2, Person3
Person1, Person2, Person3

Could someone help me explain what happens here, and especially what happens on: (persons.entries as MutableList<String>).also { it.add("Person4") }.joinToString().
Does casting with as give you a new object?

[Riccardo Lippolis]

_persons.toList() returns a new instance of a List, which contains all the elements present in _persons at the time of creation.

So when you call

(persons.entries as MutableList<String>).add("Person4")

you are adding the Person4 string to a new List instance, that is instantly lost because you don't have any more reference to it. The same applies to

(persons.entries as MutableList<String>).also { it.add("Person4") }.joinToString()

which creates a String, but does not print it.

If you do this:

println((persons.entries as MutableList<String>).also { it.add("Person4") }.joinToString())

you will see Person4 in the output. But still, it is not present the next time you call persons.entries, because that is again a new List instance

[Pim]

Makes sense now thanks a lot!