when I declare a class `MyClass` and an interface ...

# getting-started

[Manuel Pérez Alcolea]

when I declare a class MyClass and an interface MyInterface and have several classes extending and implementing them, Kotlin infers both the class and the interface in generic types. For example val list = listOf(ImplemA(), implemB()) contains objects that are MyClass and comply with MyInterface. IDEA describes the bound as <{MyClass & MyInterface}> in that case. Can I specify a type like that explicitly, without creating a new class? What's the catch?

[Dave K]

Like where?

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.

[Manuel Pérez Alcolea]

No, I know that. I'm talking about this case:

open class MyClass(val value: Int)
interface MyInterface {
    fun someMethod()
}

class ImplA : MyClass(2), MyInterface {
    override fun someMethod() {
        println("I am A")
    }
}

class ImplB : MyClass(5), MyInterface {
    override fun someMethod() {
        println("I am B")
    }
}

fun main() {
    val impls = listOf(ImplA(), ImplB()) // this a List<{MyClass & MyInterface}>

    for (ele in impls) {
        ele.someMethod()
        println("Its value is ${ele.value}")
    }
}

[Manuel Pérez Alcolea]

this is valid code, and will print

I am A
Its value is 2
I am B
Its value is 5

But it works because Kotlin knows every element in the list is MyClass and MyInterface. What if I want to have that explicitly in the code?

[Manuel Pérez Alcolea]

and without writing a weird wrapper, ofc. Is that even possible?

Ok I'm gonna go ahead and say it's probably not possible and all Kotlin's doing is just being very smart at casting for me, since this is just a List in the JVM due to type erasure

[dmitriy.novozhilov]

You can not declare intersection type explicitly. It is not-denotable type (e.g. like String! from java) and such types can be produced only by type inference
https://youtrack.jetbrains.com/issue/KT-13108