generics.md

As in Java, classes in Kotlin can have type parameters:

class Box<T>(t: T) {
    var value = t
}

In general, to create an instance of such a class, provide the type arguments:

val box: Box<Int> = Box<Int>(1)

But if the parameters may be inferred, for example, from the constructor arguments or by some other means, you can omit the type arguments:

val box = Box(1) // 1 has type Int, so the compiler figures out that it is Box<Int>

Variance

One of the most tricky parts of Java's type system is wildcard types (see Java Generics FAQ). And Kotlin doesn't have any. Instead, it has two other things: declaration-site variance and type projections.

First, let's think about why Java needs those mysterious wildcards. The problem is explained in Effective Java, 3rd Edition, Item 31: Use bounded wildcards to increase API flexibility. First, generic types in Java are invariant, meaning that List<String> is not a subtype of List<Object>. Why so? If List was not invariant, it would have been no better than Java's arrays, since the following code would have compiled and caused an exception at runtime:

// Java
List<String> strs = new ArrayList<String>();
List<Object> objs = strs; // !!! A compile-time error here saves us from a runtime exception later.
objs.add(1); // Put an Integer into a list of Strings
String s = strs.get(0); // !!! ClassCastException: Cannot cast Integer to String

So, Java prohibits such things in order to guarantee run-time safety. But this has some implications. For example, consider the addAll() method from Collection interface. What's the signature of this method? Intuitively, you'd put it this way:

// Java
interface Collection<E> ... {
    void addAll(Collection<E> items);
}

But then, you are not able to do the following simple thing (which is perfectly safe):

// Java
void copyAll(Collection<Object> to, Collection<String> from) {
    to.addAll(from);
    // !!! Would not compile with the naive declaration of addAll:
    // Collection<String> is not a subtype of Collection<Object>
}

(In Java, you learned this lesson the hard way, see Effective Java, 3rd Edition, Item 28: Prefer lists to arrays)

That's why the actual signature of addAll() is the following:

// Java
interface Collection<E> ... {
    void addAll(Collection<? extends E> items);
}

The wildcard type argument ? extends E indicates that this method accepts a collection of objects of E or some subtype of E, not just E itself. This means that you can safely read E's from items (elements of this collection are instances of a subclass of E), but cannot write to it since you don't know what objects comply to that unknown subtype of E. In return for this limitation, you have the desired behaviour: Collection<String> is a subtype of Collection<? extends Object>. In other words, the wildcard with an extends-bound (upper bound) makes the type covariant.

The key to understanding why this trick works is rather simple: if you can only take items from a collection, then using a collection of Strings and reading Objects from it is fine. Conversely, if you can only put items into the collection, it's OK to take a collection of Objects and put Strings into it: there is List<? super String> in Java, a supertype of List<Object>.

The latter is called contravariance, and you can only call methods that take String as an argument on List<? super String> (for example, you can call add(String) or set(int, String)). If you call something that returns T in List<T>, you don't get a String, but an Object.

Joshua Bloch calls those objects you only read from Producers, and those you only write to Consumers. He recommends:

"For maximum flexibility, use wildcard types on input parameters that represent producers or consumers", and proposes the following mnemonic:

PECS stands for Producer-Extends, Consumer-Super.

If you use a producer-object, say, List<? extends Foo>, you are not allowed to call add() or set() on this object, but this does not mean that this object is immutable: for example, nothing prevents you from calling clear() to remove all items from the list, since clear() does not take any parameters at all.

The only thing guaranteed by wildcards (or other types of variance) is type safety. Immutability is a completely different story.

{type="note"}

Declaration-site variance

Let's suppose that there is a generic interface Source<T> that does not have any methods that take T as a parameter, only methods that return T:

// Java
interface Source<T> {
  T nextT();
}

Then, it would be perfectly safe to store a reference to an instance of Source<String> in a variable of type Source<Object> - there are no consumer-methods to call. But Java does not know this, and still prohibits it:

// Java
void demo(Source<String> strs) {
  Source<Object> objects = strs; // !!! Not allowed in Java
  // ...
}

To fix this, you should declare objects of type Source<? extends Object>, which is meaningless, because you can call all the same methods on such a variable as before, so there's no value added by the more complex type. But the compiler does not know that.

In Kotlin, there is a way to explain this sort of thing to the compiler. This is called declaration-site variance: you can annotate the type parameter T of Source to make sure that it is only returned (produced) from members of Source<T>, and never consumed. To do this, use the out modifier:

interface Source<out T> {
    fun nextT(): T
}

fun demo(strs: Source<String>) {
    val objects: Source<Any> = strs // This is OK, since T is an out-parameter
    // ...
}

The general rule is: when a type parameter T of a class C is declared out, it may occur only in out-position in the members of C, but in return C<Base> can safely be a supertype of C<Derived>.

In other words, you can say that the class C is covariant in the parameter T, or that T is a covariant type parameter. You can think of C as being a producer of T's, and NOT a consumer of T's.

The out modifier is called a variance annotation, and since it is provided at the type parameter declaration site, it provides declaration-site variance. This is in contrast with Java's use-site variance where wildcards in the type usages make the types covariant.

In addition to out, Kotlin provides a complementary variance annotation: in. It makes a type parameter contravariant: it can only be consumed and never produced. A good example of a contravariant type is Comparable:

interface Comparable<in T> {
    operator fun compareTo(other: T): Int
}

fun demo(x: Comparable<Number>) {
    x.compareTo(1.0) // 1.0 has type Double, which is a subtype of Number
    // Thus, you can assign x to a variable of type Comparable<Double>
    val y: Comparable<Double> = x // OK!
}

The words in and out seem to be self-explaining (as they were successfully used in C# for quite some time already), thus the mnemonic mentioned above is not really needed, and one can rephrase it for a higher purpose:

The Existential Transformation: Consumer in, Producer out! :-)

Type projections

Use-site variance: type projections

It is very convenient to declare a type parameter T as out and avoid trouble with subtyping on the use site, but some classes can't actually be restricted to only return T's! A good example of this is Array:

class Array<T>(val size: Int) {
    operator fun get(index: Int): T { ... }
    operator fun set(index: Int, value: T) { ... }
}

This class cannot be either co- or contravariant in T. And this imposes certain inflexibilities. Consider the following function:

fun copy(from: Array<Any>, to: Array<Any>) {
    assert(from.size == to.size)
    for (i in from.indices)
        to[i] = from[i]
}

This function is supposed to copy items from one array to another. Let's try to apply it in practice:

val ints: Array<Int> = arrayOf(1, 2, 3)
val any = Array<Any>(3) { "" } 
copy(ints, any)
//   ^ type is Array<Int> but Array<Any> was expected

Here you run into the same familiar problem: Array<T> is invariant in T, thus neither of Array<Int> and Array<Any> is a subtype of the other. Why? Again, because copy may have an unexpected behavior, for example, it may attempt to write a String to from, and if you actually pass an array of Int there, a ClassCastException'll be thrown sometime later.

To prohibit the copy function from writing to from, do the following:

fun copy(from: Array<out Any>, to: Array<Any>) { ... }

This is type projection, that means that from is not simply an array, but a restricted (projected) one. You can only call those methods that return the type parameter T, in this case it means that you can only call get(). This is our approach to use-site variance, and corresponds to Java's Array<? extends Object>, but in a slightly simpler way.

You can project a type with in as well:

fun fill(dest: Array<in String>, value: String) { ... }

Array<in String> corresponds to Java's Array<? super String>. This means that you can pass an array of CharSequence or an array of Object to the fill() function.

Star-projections

Sometimes you want to say that you know nothing about the type argument, but still want to use it in a safe way. The safe way here is to define such a projection of the generic type, that every concrete instantiation of that generic type would be a subtype of that projection.

Kotlin provides so called star-projection syntax for this:

• For Foo<out T : TUpper>, where T is a covariant type parameter with the upper bound TUpper, Foo<*> is equivalent to Foo<out TUpper>. It means that when the T is unknown you can safely read values of TUpper from Foo<*>.
• For Foo<in T>, where T is a contravariant type parameter, Foo<*> is equivalent to Foo<in Nothing>. It means there is nothing you can write to Foo<*> in a safe way when T is unknown.
• For Foo<T : TUpper>, where T is an invariant type parameter with the upper bound TUpper, Foo<*> is equivalent to Foo<out TUpper> for reading values and to Foo<in Nothing> for writing values.

If a generic type has several type parameters each of them can be projected independently. For example, if the type is declared as interface Function<in T, out U> you can imagine the following star-projections:

• Function<*, String> means Function<in Nothing, String>.
• Function<Int, *> means Function<Int, out Any?>.
• Function<*, *> means Function<in Nothing, out Any?>.

Star-projections are very much like Java's raw types, but safe.

{type="note"}

Generic functions

Not only classes can have type parameters. Functions can, too. Type parameters are placed before the name of the function:

fun <T> singletonList(item: T): List<T> {
    // ...
}

fun <T> T.basicToString(): String {  // extension function
    // ...
}

To call a generic function, specify the type arguments at the call site after the name of the function:

val l = singletonList<Int>(1)

Type arguments can be omitted if they can be inferred from the context, so the following example works as well:

val l = singletonList(1)

Generic constraints

The set of all possible types that can be substituted for a given type parameter may be restricted by generic constraints.

Upper bounds

The most common type of constraint is an upper bound that corresponds to Java's extends keyword:

fun <T : Comparable<T>> sort(list: List<T>) {  ... }

The type specified after a colon is the upper bound: only a subtype of Comparable<T> can be substituted for T. For example:

sort(listOf(1, 2, 3)) // OK. Int is a subtype of Comparable<Int>
sort(listOf(HashMap<Int, String>())) // Error: HashMap<Int, String> is not a subtype of Comparable<HashMap<Int, String>>

The default upper bound (if none specified) is Any?. Only one upper bound can be specified inside the angle brackets. If the same type parameter needs more than one upper bound, you need a separate where-clause:

fun <T> copyWhenGreater(list: List<T>, threshold: T): List<String>
    where T : CharSequence,
          T : Comparable<T> {
    return list.filter { it > threshold }.map { it.toString() }
}

The passed type must satisfy all conditions of the where clause simultaneously. In the above example, the T type must implement both CharSequence and Comparable.

Type erasure

The type safety checks that Kotlin performs for generic declaration usages are only done at compile time. At runtime, the instances of generic types do not hold any information about their actual type arguments. The type information is said to be erased. For example, the instances of Foo<Bar> and Foo<Baz?> are erased to just Foo<*>.

Therefore, there is no general way to check whether an instance of a generic type was created with certain type arguments at runtime, and the compiler prohibits such is-checks.

Type casts to generic types with concrete type arguments, for example, foo as List<String>, cannot be checked at runtime.
These unchecked casts can be used when type safety is implied by the high-level program logic but cannot be inferred directly by the compiler. The compiler issues a warning on unchecked casts, and at runtime, only the non-generic part is checked (equivalent to foo as List<*>).

The type arguments of a generic function calls are also only checked at compile time. Inside the function bodies, the type parameters cannot be used for type checks, and type casts to type parameters (foo as T) are unchecked. However, reified type parameters of inline functions are substituted by the actual type arguments in the inlined function body at the call sites and thus can be used for type checks and casts, with the same restrictions for instances of generic types as described above.