Selgitage invariantsuse kontseptsiooni Kotlin'i generikates ja miks seda kasutatakse.

Invariance in Kotlin means that if you have a generic type Generic<T>, then Generic<A> and Generic<B> are not related by inheritance, even if A is a subtype of B. In other words, Generic<A> is not a subtype of Generic<B>, and vice versa.

This is important to avoid type errors when working with mutable data structures. For example, if MutableList<String> were a subtype of MutableList<Any>, then it would be possible to add an object of a different type to the list, which would violate type safety.

In Kotlin, invariance is the default for mutable collections and other types where the content can change. For greater flexibility, covariance (out) and contravariance (in) exist, allowing safe extension or narrowing of types in certain cases.

Example of an invariant type:

class Box<T>(var value: T)

val boxString: Box<String> = Box("text")
// val boxAny: Box<Any> = boxString // Compilation error: Box<String> is not a subtype of Box<Any>

Thus, invariance helps maintain strict type safety when working with generic types.