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Understanding Kotlin Covariance and Contravariance

Introduction

To someone coming from the Java world to the world of Kotlin, the concepts of Covariance and Contravariance can be somewhat overwhelming to grab at first. But they don’t have to be once you start understanding the basic cadence of it. This article is going to be a step by step depiction of how I have come to understand these two topics.


Table of Contents


Let’s create some classes!

First let’s create a small class hierarchy that we can use to understand these concepts. Paper is the parent class of the Regular paper and the Premium paper.

open class Paper {

}

class Regular: Paper() {

}

class Premium: Paper() {

}

Let’s create some lists!!

First let’s create a List of Regular papers called regularPapers.

val regularPapers: List<Regular> = listOf(Regular())

And now if we declare a List of the parent type Paper called papers and try to assign the just created regularPapers List to it, we will see that it works without any compilation errors.

val papers: List<Paper> = regularPapers

Sure a List of Regular paper is still a List of Paper, you say. There is nothing earth-shattering about it!

But when you look at the following example you will surely think that there is more going on under the hood than meets the eye.

    val mutableRegularPapers: MutableList<Regular> = mutableListOf(Regular())
    val mutablePapers: MutableList<Paper> = mutableRegularPapers //This line gives compilation error 

When you replace the List with a MutableList in our example, you will see that the compilation fails with this error: Type mismatch: inferred type is MutableList<Regular> but MutableList<Paper> was expected.

But what could be so different between a List and a MutableList so that a List of Regular papers can be used as a List of Paper but a MutableList of Regular papers can not be used as a MutableList of Paper? As it turns out, the answer lies in the definitions of List and MutableList.

public interface List<out E> : Collection<E> {
  ..
}

public interface MutableList<E> : List<E>, MutableCollection<E> {
  ..
}

The out keyword in the List declaration makes all the difference. This means that a List<T> is assignable to any List<U> where U is a subtype of T. But the same is not true for a MutableList since it’s declared as MutableList<E> and not as MutableList<out E>. This is Covariance for you in a nutshell.

Covariance

Covariance follows the natural inheritance order where a less generic subtype can be used in place of a more generic supertype. Covariant is expressed with the out keyword.

Let’s declare a Covariant class called Box.

class Box<out T> {
    
}

Now we can create a Box of Regular papers and assign it to a Box of Paper. The following code compiles without any error.

val regularPaperBox = Box<Regular>()
val paperBox: Box<Paper> = regularPaperBox

So Covariance is the Kotlin Generics feature that allows us to pass a Box of Regular papers as a Box of Paper.

Contravariance

Contravariance is the inverse of Covariance where a more generic subtype can be used in place of a less generic supertype. Contravariance is expressed with the in keyword.

Let’s declare a Contravariant class called Printer.

class Printer<in T> {
    
}

When your office Printer that uses Premium papers is out of order, it may be okay to have a temporary replacement Printer that uses any Paper. The following code compiles without any error.

val paperPrinter = Printer<Paper>()
val premiumPrinter: Printer<Premium> = paperPrinter

So Contravariance is the Kotlin Generics feature that allows us to use a Printer of parent type Paper in place of a Printer of Premium papers.

Limitations of Covariance and Contravariance

Covariance and Contravariance do not come without their limitations over the Invariant types (types that do not use either of the out or in keywords and are rigid in terms of accepting either the subtypes or the supertypes) generic types.

A Class using a Covariant type can only be used as a Producer of the Covariant type, never a Consumer. What this means is that our Box class can only have functions that output T but never a function that takes T as an input.

class Box<out T> {
    fun take(): T { //OK!
      ...
    }
    fun put(t: T) {} //This line does not compile
}

This is not an unreasonable ask when you think about it from the perspective of the compiler - given that the generics type information is erased during the compilation and is unavailable in the runtime, if we could somehow use a Covariant type as an input to the add function of a MutableList, there would have been nothing to stop us from inserting a Premium paper in a list of purely Regular papers which could have caused all types of issues afterwards.

val regularPapers: List<Regular> = listOf(Regular())
val papers: List<Paper> = regularPapers
papers.add(Premium()) //This line does not compile, thankfully!

Similarly a Class using a Contravariant type can only be used as a Consumer of the Contravariant type, never a Producer. In other words, our Printer class can only have functions that take T as inputs but never a function that outputs T.

class Printer<in T> {
    fun take(t: T) {} //OK!

    fun eject(): T { //This line does not compile!
    }
}

Which is again understandable since the Contravariant mutableRegularPapers has no way to ensure in the runtime that the paper it returns is of type only Regular.

val mutablePapers: MutableList<Paper> = mutableListOf(Paper())
val mutableRegularPapers: MutableList<in Regular> = mutablePapers
val regular: Regular = mutableRegularPapers.removeAt(0) //This line does not compile as the mutableRegularPapers.removeAt(0) returns an object of type Any?

Conclusion

The Covariant and Contravariant examples used in this article are called Declaration-site variance as they are used in Class or Interface declaration. When Covariant and Contravariant types are used in function declaration, it’s called Use-site variance. Although both work in the same way, it is important to know the distinction. Even though Kotlin supports both, Java only supports the latter.



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