Android Development with Kotlin

Use-site variance versus declaration-site variance

Use-site variance and declaration-site variance basically describes the place in the code (site) where the variance modifier is specified. Let's consider a View and Presenter example:

    interface BaseView    interface ProductView : BaseView    class Presenter<T>    // Usage    var preseter = Presenter<BaseView>()    var productPresenter = Presenter<ProductView>()    preseter = productPresenter    // Error: Type mismatch    // Required: Presenter<BaseView>    // Found: Presenter<ProductView>

The Presenter class is invariant on its parameterT type. To fix the problem, we can explicitly define the subtyping relationship. We can do it in two ways (use-site and declaration-site). First, let's define the variance at the use-site: ...