Still learning about generics In this example I would expect

Question

Still learning about generics In this example I would expect this to work because I feel like `Animal lt Int gt ` fits into `Animal lt Any gt ` What am I missing ```open class Animal lt T gt class Dog Animal lt Int gt val x Animal lt Any gt = Dog error at the constructor call```

Joffrey · Answer

By default generics are invariant which means there is no subtyping relations between different `Animal lt T gt ` types even if there is a subtype relation between the `T` inside So what you re seeing is expected If you want `Animal lt Int gt ` to be a subtype of `Animal lt Any gt ` you need to make `Animal lt T gt ` covariant in `T` This is done by adding the keyword `out` in front of `T`

poohbar · Answer

```open class Animal lt out T gt class Dog Animal lt Int gt val x Animal lt Any gt = Dog works```

poohbar · Answer

i think i get it it is restrictive in some cases where i want to just escape Kotlin s type system and go fully dynamic something like using `List` in Java I don t think it s possible in Kotlin thinking face

Jacob · Answer

What does an animal do with Ts Do they have a method that returns them then it makes sense that a dog is an animal lt any gt since it returns Ints which are techincally also Anys Do animals eat Ts ie have functions that take T as an arg Than a dog is not an animal lt any gt since it doesn t eat anything dogs only eat ints

Jacob · Answer

You can go fully Dynamic just like java ```open class Animal class Dog Animal val x Animal = Dog works``` laughing

poohbar · Answer

i know i know in most cases the generics are very helpful which is why i dont want to just remove them

Joffrey · Answer

Kotlin doesn t have Java s raw types which are pretty bad but maybe you are looking for the syntax `Animal lt gt `

ephemient · Answer

`List lt out E gt ` is how it s declared in Kotlin because an unmodifiable list is covariant Java s `List lt E gt ` is mutable thus the only reasonable behavior for the generic is invariant Kotlin does the same with `MutableList lt E gt ` Java treats arrays as covariant for historical reasons but that s very broken see <https docs oracle com javase 8 docs api java lang ArrayStoreException html>

Vampire · Answer

Java also just allows raw types for backwards compatibility Any code that uses them intentionally should be considered broken and wrong

Joffrey · Answer

gt Any code that uses them intentionally should be considered broken and wrong We should make a poster or T shirt out of this smile You should see some of the code I found in my previous company full of raw types and generics were used in ways that were just here to fool the compiler to make it at least compile

Vampire · Answer

Most codebases probably have those smile Doesn t make it better though see no evil

Jacob · Answer

On the topic of fooling the compiler take a look at how mockito kotlin can create instances of anything <https github com mockito mockito kotlin blob 37bbae48634d879640a754a1b122db99f0b00478 mockito kotlin src main kotlin org mockito kotlin internal CreateInstance kt L56|https github com mockito mockito kotlin blob 37bbae48634d879640a754a1b122db99f0b n src main kotlin org mockito kotlin internal CreateInstance kt>

ephemient · Answer

I m not sure I d call Kotlin s star projection a raw type it doesn t automatically cast into the generic type and it exposes a ` lt gt ` signature to Java

Joffrey · Answer

I don t believe anyone called the Kotlin star projection a raw type What are you referring to

ephemient · Answer

oh I misread Björn s reply

Vampire · Answer

Definitely didn t intend to imply they are similar slightly smiling face