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Generics

This material was written by Aasmund Eldhuset; it is owned by Khan Academy and is licensed for use under CC BY-NC-SA 3.0 US. Please note that this is not a part of Khan Academy's official product offering.


Generic type parameters

One might think that static typing would make it very impractical to make collection classes or any other class that needs to contain members whose types vary with each usage. Generics to the rescue: they allow you to specify a "placeholder" type in a class or function that must be filled in whenever the class or function is used. For example, a node in a linked list needs to contain data of some type that is not known when we write the class, so we introduce a generic type parameter T (they are conventionally given single-letter names):

class TreeNode<T>(val value: T?, val next: TreeNode<T>? = null)

Whenever you create an instance of this class, you must specify an actual type in place of T, unless the compiler can infer it from the constructor parameters: TreeNode("foo") or TreeNode<String>(null). Every use of this instance will act as if it were an instance of a class that looks like this:

class TreeNode<String>(val value: String?, val next: TreeNode<String>? = null)

Member properties and member functions inside a generic class may for the most part use the class' generic type parameters as if they were ordinary types, without having to redeclare them. It is also possible to make functions that take more generic parameters than the class does, and to make generic functions inside nongeneric classes, and to make generic top-level functions (which is what we'll do in the next example). Note the different placement of the generic type parameter in generic function declarations:

fun <T> makeLinkedList(vararg elements: T): TreeNode<T>? {
    var node: TreeNode<T>? = null
    for (element in elements.reversed()) {
        node = TreeNode(element, node)
    }
    return node
}

Constraints

You can restrict the types that can be used for a generic type parameter, by specifying that it must be an instance of a specific type or of a subclass thereof. If you've got a class or interface called Vehicle, you can do:

class TreeNode<T : Vehicle>

Now, you may not create a TreeNode of a type that is not a subclass/implementor of Vehicle. Inside the class, whenever you've got a value of type T, you may access all the public members of Vehicle on it.

If you want to impose additional constraints, you must use a separate where clause, in which case the type parameter must be a subclass of the given class (if you specify a class, and you can specify at most one) and implement all the given interfaces. You may then access all the public members of all the given types whenever you've got a value of type T:

class TreeNode<T> where T : Vehicle, T : HasWheels

Variance

Introduction

Pop quiz: if Apple is a subtype of Fruit, and Bowl is a generic container class, is Bowl<Apple> a subtype of Bowl<Fruit>? The answer is - perhaps surprisingly - no. The reason is that if it were a subtype, we would be able to break the type system like this:

fun add(bowl: Bowl<Fruit>, fruit: Fruit) = bowl.add(fruit)

val bowl = Bowl<Apple>()
add(bowl, Pear()) // Doesn't actually compile!
val apple = bowl.get() // Boom!

If the second-to-last line compiled, it would allow us to put a pear into what is ostensibly a bowl of only apples, and your code would explode when it tried to extract the "apple" from the bowl. However, it's frequently useful to be able to let the type hierarchy of a generic type parameter "flow" to the generic class. As we saw above, though, some care must be taken - the solution is to restrict the direction in which you can move data in and out of the generic object.

Declaration-site covariance and contravariance

If you have an instance of Generic<Subtype>, and you want to refer to it as a Generic<Supertype>, you can safely get instances of the generic type parameter from it - these will truly be instances of Subtype (because they come from an instance of Generic<Subtype>), but they will appear to you as instances of Supertype (because you've told the compiler that you have a Generic<Supertype>). This is safe; it is called covariance, and Kotlin lets you do declaration-site covariance by putting out in front of the generic type parameter. If you do, you may only use that type parameter as a return type, not as a parameter type. Here is the simplest useful covariant interface:

interface Producer<out T> {
    fun get(): T
}

It is safe to treat a Producer<Apple> as if it were a Producer<Fruit> - the only thing it will ever produce is Apple instances, but that's okay, because an Apple is a Fruit.

Conversely, if you have an instance of Generic<Supertype>, and you want to refer to it as a Generic<Subtype> (which you can't do with nongeneric classes), you can safely give instances of the generic type parameter to it - the compiler will require those instances to be of the type Subtype, which will be acceptable to the real instance because it can handle any Supertype. This is called contravariance, and Kotlin lets you do declaration-site contravariance by putting in in front of the generic type parameter. If you do, you may only use that type parameter as a parameter type, not as a return type. Here is the simplest useful contravariant interface:

interface Consumer<in T> {
    fun add(item: T)
}

It is safe to treat a Consumer<Fruit> as a Consumer<Apple> - you are then restricted to only adding Apple instances to it, but that's okay, because it is capable of receiving any Fruit.

With these two interfaces, we can make a more versatile fruit bowl. The bowl itself needs to both produce and consume its generic type, so it can neither be covariant nor contravariant, but it can implement our covariant and contravariant interfaces:

class Bowl<T> : Producer<T>, Consumer<T> {
    private val items = mutableListOf<T>()
    override fun get(): T = items.removeAt(items.size - 1)
    override fun add(item: T) { items.add(item) }
}

Now, you can treat a bowl of T as a producer of any superclass of T, and as a consumer of any subclass of T:

val p: Producer<Fruit> = Bowl<Apple>()
val c: Consumer<Apple> = Bowl<Fruit>()

Variance directions

If the parameters or return types of the members of a variant type are themselves variant, it gets a bit complicated. Function types in parameters and return types make it even more challenging. If you're wondering whether it's safe to use a variant type parameter T in a particular position, ask yourself:

  • If T is covariant: is it okay that the user of my class thinks that T in this position is a Supertype, while in reality, it's a Subtype?
  • If T is contravariant: is it okay that the user of my class thinks that T in this position is a Subtype, while in reality, it's a Supertype?

These considerations lead to the following rules. A covariant type parameter T (which the user of an object might think is Fruit, while the object in reality is tied to Apple) may be used as:

  • val v: T

    A read-only property type (the user is expecting a Fruit, and gets an Apple)

  • val p: Producer<T>

    The covariant type parameter of a read-only property type (the user is expecting a producer of Fruit, and gets a producer of Apple)

  • fun f(): T

    A return type (as we've already seen)

  • fun f(): Producer<T>

    The covariant type parameter of a return type (the user is expecting that the returned value will produce a Fruit, so it's okay if it really produces an Apple)

  • fun f(consumer: Consumer<T>)

    The contravariant type parameter of a parameter type (the user is passing a consumer that can handle any Fruit, and it will be given an Apple)

  • fun f(function: (T) -> Unit)

    The parameter type of a function-typed parameter (the user is passing a function that can handle any Fruit, and it will be given an Apple)

  • fun f(function: (Producer<T>) -> Unit)

    The covariant type parameter of the parameter type of a function-typed parameter (the user is passing a function that can handle any Fruit producer, and it will be given an Apple producer)

  • fun f(function: () -> Consumer<T>)

    The contravariant type parameter of the return type of a function-typed parameter (the user is passing a function that will return a consumer of any Fruit, and the returned consumer will be given Apple instances)

  • fun f(): () -> T

    The return type of a function-typed return type (the user expects the returned function to return Fruit, so it's okay if it really returns Apple)

  • fun f(): () -> Producer<T>

    The covariant type parameter of the return type of a function-typed return type (the user expects the returned function to return something that produces Fruit, so it's okay if it really produces Apple)

  • fun f(): (Consumer<T>) -> Unit

    The contravariant type parameter of a parameter of a function-typed return type (the user will call the returned function with something that can consume any Fruit, so it's okay to return a function that expects to receive something that can handle Apple)

A contravariant type parameter may be used in the converse situations. It is left as an exercise to the reader to figure out the justifications for why these member signatures are legal:

  • val c: Consumer<T>
  • fun f(item: T)
  • fun f(): Consumer<T>
  • fun f(producer: Producer<T>)
  • fun f(function: () -> T)
  • fun f(function: () -> Producer<T>)
  • fun f(function: (Consumer<T>) -> Unit)
  • fun f(): (T) -> Unit
  • fun f(): (Producer<T>) -> Unit
  • fun f(): () -> Consumer<T>

Type projections (use-site covariance and contravariance)

If you're using a generic class whose type parameters haven't been declared in a variant way (either because its authors didn't think of it, or because the type parameters can't have either variance kind because they are used both as parameter types and return types), you can still use it in a variant way thanks to type projection. The term "projection" refers to the fact that when you do this, you might restrict yourself to using only some of its members - so you're in a sense only seeing a partial, or "projected" version of the class. Let's look again at our Bowl class, but without the variant interfaces this time:

class Bowl<T> {
    private val items = mutableListOf<T>()
    fun get(): T = items.removeAt(items.size - 1)
    fun add(item: T) { items.add(item) }
}

Because T is used as a parameter type, it can't be covariant, and because it's used as a return type, it can't be contravariant. But if we only want to use the get() function, we can project it covariantly with out:

fun <T> moveCovariantly(from: Bowl<out T>, to: Bowl<T>) {
    to.add(from.get())
}

Here, we're saying that the type parameter of from must be a subtype of the type parameter of to. This function will accept e.g. a Bowl<Apple> as from and Bowl<Fruit> as to. The price we're paying for using the out projection is that we can't call add() on from(), since we don't know its true type parameter and we would therefore risk adding incompatible fruits to it.

We could do a similar thing with contravariant projection by using in:

fun <T> moveContravariantly(from: Bowl<T>, to: Bowl<in T>) {
    to.add(from.get())
}

Now, the type parameter of to must be a supertype of that of from. This time, we're losing the ability to call get() on to.

The same type parameter can be used in both covariant and contravariant projections (because it's the generic classes that are being projected, not the type parameter):

fun <T> moveContravariantly(from: Bowl<out T>, to: Bowl<in T>) {
    to.add(from.get())
}

While doing so was not useful in this particular example, one could get interesting effects by adding an unprojected parameter type via: Bowl<T>, in which case the generic type parameter of via would be forced to be "in-between" those of from and to.

If you don't have any idea (or don't care) what the generic type might be, you can use a star-projection:

fun printSize(items: List<*>) = println(items.size)

When using a generic type where you have star-projected one or more of its type parameters, you can:

  • Use any members that don't mention the star-projected type parameter(s) at all
  • Use any members that return the star-projected type parameter(s), but the return type will appear to be Any? (unless the type parameter is constrained, in which case you'll get the type mentioned in the constraint)
  • Not use any members that take a star-projected type as a parameter

Reified type parameters

Sadly, Kotlin has inherited Java's limitation on generics: they are strictly a compile-time concept - the generic type information is erased at runtime. Therefore, you can not say T() to construct a new instance of a generic type; you can not at runtime check if an object is an instance of a generic type parameter; and if you try to cast between generic types, the compiler can't guarantee the correctness of it.

Luckily, Kotlin has got reified type parameters, which alleviates some of these problems. By writing reified in front of a generic type parameter, it does become available at runtime, and you'll get to write T::class to get the class metadata. You can only do this in inline functions (because an inline function will be compiled into its callsite, where the type information is available at runtime), but it still goes a long way. For example, you can make an inline wrapper function for a big function that has got a less elegant signature.

In the example below, we assume that there is a DbModel base class, and that every subclass has got a parameterless primary constructor. In the inline function, T is reified, so we can get the class metadata. We pass this to the function that does the real work of talking to the database.

inline fun <reified T : DbModel> loadFromDb(id: String): T =
    loadFromDb(T::class, id)

fun <T : DbModel> loadFromDb(cls: KClass<T>, id: String): T {
    val entity = cls.primaryConstructor!!.call()
    val tableName = cls.simpleName
    // DB magic goes here - load from table `tableName`,
    // and use the data to populate `entity`
    // (possibly via `memberProperties`)
    return entity
}

Now, you can say loadFromDb<Exercise>("x01234567") to load an object from the Exercise database table.


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