Collection magic

4 min read


By Jørn Ola Birkeland


December 22, 2020

Kotlin has a restricted menu of collection types available. Basically, it boils down to Set, List, and Map (even though Maptechnically is not a Collection), whereas java seemingly has a much larger lineup. For example, java's List interface has implementations like Stack, Vector and LinkedList. On top of that, the kotlin collection interfaces have less features than their java counterparts.

One rationale behind this design choice is probably platform interoperability - simpler interfaces would allow kotlin code to be more easily compiled down to javascript, mobile platforms, or other languages and virtual machines. Another reason is that extension methods and properties allow kotlin to add a lot more features to the various collections without "polluting" the interface. Finally, the corresponding java interfaces are all mutable, with methods like add and remove, while kotlin has explicit interfaces for mutable collections, like MutableList, MutableSet, and MutableMap. This is a cleaner, more readable alternative to the Unmodifiable* wrappers for the java.util.List interface in java.

Under the hood kotlin uses the corresponding java classes, and a bit of compiler magic to ensure that the types effectively seem to implement the kotlin interfaces, known as mapped types. Runtime, however, there is no difference between the java and kotlin counterparts. There is a couple of big benefits with this approach: first, instead of implementing new collections natively in kotlin, the battletested implementations of java can be used. Second, interoperability between java and kotlin is ensured. And it means you can still use thejava.util.Vector class, and get the extension goodies associated with the kotlin Collection and List interfaces in kotlin.

val vector = Vector<String>(3) // java.util.Vector
val first = vector.first()     // from kotlin.collections

It goes the other way, too: if you decide to make an implementation of the kotlin List interface yourself, the compiler will replace it with java.util.List, and supply the missing method implementations itself, all throwing an UnsupportedOperationException. In other words, the kotlin collection interfaces are somewhat of an illusion.

Generally, it is a good idea to use factory methods for creating collections. Examples are listOf, setOf and mapOf, and the corresponding mutableListOf, mutableSetOf and mutibleMapOf for the mutable variants. It leaves your code less dependent on concrete implementations, which may or may not be portable across platforms, and open for future improvements to the default implementation.

Using mutableListOf will in the end give you an instance of java.util.ArrayList, while listOf returns an java.util.Arrays$ArrayList - a lightweight, read-only wrapper over an array. Except if the list is empty, which in the immutable case will give you the singleton EmptyList - a nice example of a simple optimization done for you. In the same vein setOf and mapOf will return LinkedHashSet and LinkedHashMap, respectively, and exactly the same in the mutable case. The linked versions ensure that the collections will be iterated in the same order as the items were added to them, but both of these have a higher memory footprint than HashSet and HashMap would have. If you need to deal with large sets or maps, this could be an issue, and you would want to use the more specialized factory methods, like hashSetOf and hashMapOf.

An interesting observation is that since the underlying type is the same for mutable and immutable Sets and Maps, the compiler will gladly accept a change to an immutable collection

 val aSet = setOf("A", "B", "C") // Immutable

 if(aSet is MutableSet) {
    val modifiableSet: MutableSet<String> = aSet


which will print [B, C, D].

The fact that a collection is mutable, is not the same that the variable needs to be mutable. In fact, the opposite is closer to the truth. Consider

// Immutable list assigned to reassignable variable
var varList = listOf("good","better","best")
varList = listOf("even better")

// Mutable list assigned to read-only variable
val valList = mutableListOf("good","better","best")
valList.add("even better")

A mutable collection can (and should?) be assigned to a read-only variable. Even though the variable cannot be reassigned you can freely modify the collection. You would in principle expect better performance by modifying the contents of a single collection, rather than creating new instances from scratch (but is a premature optimization until you actually hit a performance problem, of course)

All in all, kotlin's handling of collections is a pragmatic compromise between interoperability, reuse, and readability. The result is robust, high-performing implementations, sleak interfaces, and an abundance of extensions to make your developer experience festive and joyful.

If you want to learn more about immutabiliy, have a look at this year's calendar's very first entry.