Kotlin collections

This post should have been the 5th in the Scala vs Kotlin serie.

Unfortunately, I must admit I have a hard time reading the documentation of Scala collections e.g.:

trait LinearSeq [+A] extends Seq[A] with collection.LinearSeq[A] with GenericTraversableTemplate[A, LinearSeq] with LinearSeqLike[A, LinearSeq[A]]

Hence, I will only describe collections from the Kotlin side.


At the root of Kotlin’s Collection API lies the Iterator interface, similar to Java’s. But the similitude stops after that. java.util.ListIterator features are broken down into different contracts:

  1. ListIterator to move the iterator index forward and backward
  2. MutableIterator to remove content from the iterator
  3. MutableListIterator inherits from the 2 interfaces above to mimic the entire contract of java.util.ListIterator
Iterator API

Collection, List and Set

The hierarchy of collections in Kotlin are very similar as in Java: Collection, List and Set. (I won’t detail maps, but they follow the same design). The only, but huge, difference is that it’s divided between mutable and immutable types. Mutable types have methods to change their contents (e.g. add() and `set()), while immutable types don’t.

Of course, the hierarchy is a bit more detailed compared to Java, but that’s expected from a language that benefits from its parent’s experience.

Collection, List and Set


IMHO, the important bit about Kotlin collections is not their hierarchy - though it’s important to understand about the difference between mutable and immutable.

As Java developers know, there’s no such things as out-of-the-box immutable collection types in Java. When an immutable collection is required, the mutable collection must be wrapped into an unmodifiable type via a call to the relevant Collections.unmodifiableXXX(). But unmodifiable types are not public, they are private in Collections: types returned are generic ones (List or Set interfaces). It means they implement all methods of the standard collections. Immutability comes from the mutable-related methods throwing exceptions: at compile time, there’s no way to differentiate between a mutable and an immutable collection.

On the opposite, Kotlin offers a clean separation between mutable and immutable types. It also provides dedicated functions to create objects of the relevant type:

Collections-creating functions

As opposed to Scala, Kotlin doesn’t implement its own collection types, it reuses those from Java. That means that even when the compile-time type is immutable, the runtime type is always mutable. The downside is that it’s possible to change the collection elements by casting it to the correct runtime type. IMHO, this is no more severe than what allows standard reflection. There are several advantages, though:

  1. Faster time-to-market
  2. Java collection types benefited from years of improvement
  3. The underlying implementation can be changed in the future with full backward compatibility
Nicolas Fränkel

Nicolas Fränkel

Developer Advocate with 15+ years experience consulting for many different customers, in a wide range of contexts (such as telecoms, banking, insurances, large retail and public sector). Usually working on Java/Java EE and Spring technologies, but with focused interests like Rich Internet Applications, Testing, CI/CD and DevOps. Also double as a trainer and triples as a book author.

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Kotlin collections
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