Kotlin generics

kotlin generics variance

Similar to what Java provisions, kotlin support generics and also variance, but is easier to understand and use.

For instance we have a class Container that accepts T is type parameter.

class Container<T>

The class Container itself can have inheritance and everything, but when creating instance, it is not the case.

val a: Container<String> = Container<String>()
val b: Container<Any> = a // compilation error here

Would result into error below:

Type mismatch.
Required: Container
Found: Container

This is because Container<Any> is not the parent class of Container<String>.

But in reality, there are many situations that we would want our class to behave as a generic container to deal with certain types and its inheritance, there comes the variance conception.

in

This is to denote the generic type can only be used as input parameter, but can never return.

class Container<in T> {
  fun add(data: T) {}
}

val a = Container<Any>()
a.add("string")
a.add(1 to 2)

Here the class Container is used as consumer of type T, thus any type that extends T can be passed as function parameter, but never as return value.

out

class Container<out T> {
  fun get(): T {}
}

val a = Container<String>()
val b: String = a.get()
val c: Any = a.get()

This time, Container class is used as producer, it may return any super class of T, hence the return value can be assigned to the variable b and c.