ReadOnlyProperty - type arguments automatically inferred not validating bounds

user_6892385 0
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I'm trying to inject value into class property using by keyword.
No problem implementing the code below.

open class Component
class ComponentA: Component()

class Equipment(val components: List<Component>) {

    inline fun <reified T> getComponent(): T? {
        return components.firstOrNull { it is T } as T?
    }

    val ca: ComponentA by find<ComponentA>()  //this is what I intend to do
    val ca1: List<ComponentA> by find()       //no error or warning
    val ca2 by find<List<ComponentA>>()       //Error: Type argument is not within its bounds
}

inline fun <reified T> find(): ReadOnlyProperty<Equipment, T> where T : Component {
    return ReadOnlyProperty { ref, _ -> ref.getComponent<T>() ?: throw Exception("Not found") }
}

But I find it is weird that the val ca1: List<ComponentA> by find() not showing any error or warning. Error still show normally when I try val ca2 by find<List<ComponentA>>().
Can anybody explain why this happened and how to make sure ca1 definition syntax is invalid.

I'm using:

  • AdoptOpenJDK 11.0.7.10 hotspot
  • Kotlin 1.6.21
  • IntelliJ IDEA 2021.3.2
kotlin syntax
1 Answers

well I think the problem is from IDEA with the compiler, I tried using android functions like viewModels() with List instead of ViewModel and I had the same problem, the IDEA will not show any error:

enter image description here

but when you compile the code you will get an error:

enter image description here

so you can ignore this, it's just a problem with the IDEA, also you have some mistakes in your code,

val components: List<Equipment> this should be val components: List< Component >

return components.firstOrNull { it is T } as T and this should be return components.firstOrNull { it is T } as T? because you cannot cast a null type to a non-null type

so your code should look like this:

open class Component
class ComponentA: Component()

class Equipment(val components: List<Component>) {

    inline fun <reified T> getComponent(): T? {
        return components.firstOrNull { it is T } as T?
    }

    val ca: ComponentA by find<ComponentA>()  //this is what I intend to do
    // val ca1: List<ComponentA> by find()       //no error or warning
    // val ca2 by find<List<ComponentA>>()       //Error: Type argument is not within its bounds
}

inline fun <reified T> find(): ReadOnlyProperty<Equipment, T> where T : Component {
    return ReadOnlyProperty { ref, _ -> ref.getComponent<T>() ?: throw Exception("Not found") }
}
user_116177540
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