Defensive copy of a mutable collection in Kotlin data class

Question

I want to have a data class accepting a read-only list:

data class Notebook(val notes: List<String>) {
}

But it can accept MutableList as well, because it is a subtype of the List.

For example the following code modifies the passed in list:

fun main(args: Array<String>) {
    val notes = arrayListOf("One", "Two")
    val notebook = Notebook(notes)

    notes.add("Three")

    println(notebook)       // prints: Notebook(notes=[One, Two, Three])
}

Is there a way how perform defensive copy of the passed in list in the data class?

Answer 1

I think better is to use JetBrains library for immutable collections - https://github.com/Kotlin/kotlinx.collections.immutable

Import in your project

Add the bintray repository:

repositories {
    maven {
        url "http://dl.bintray.com/kotlin/kotlinx"
    }
}

Add the dependency:

compile 'org.jetbrains.kotlinx:kotlinx-collections-immutable:0.1'

Result:

data class Notebook(val notes: ImmutableList<String>) {}

fun main(args: Array<String>) {
    val notes = immutableListOf("One", "Two")
    val notebook = Notebook(notes)

    notes.add("Three") // creates a new collection

    println(notebook)       // prints: Notebook(notes=[One, Two])
}

Note: You also can use add and remove methods with ImmutableList, but this methods doesn't modify current list just creates new one with your changes and return it for you.