Sealed class explanation

Language Design
1

I have been reading about sealed classes and don’t really get why they would be needed.

Does anyone have a good example to explain how you would use them?

2

Have you checked the example given in the official documentation? What’s unclear about it?

3

The official example was too brief. That applies to a lot of the documentation actually, there is a lot of assumed knowledge.

I found a great example which explained it better at https://blog.simon-wirtz.de/sealed-classes-kotlin-explanation/ if anyone is confused by sealed classes.

4

How familiar are you with Java? One challenge is the Kotlin is starting to attract developers who have primary experience outside Java. It is a problem for our team as we have no java projects, only kotlin projects, and Kotlin documentation tends to assume java knowledge. Other than train devs on a language they will not learn, there is no path for learning kotlin. We have scaled back on kotlin for this reason.

5

The purpose is for you to know all possible direct subtypes of a type.
So, for example, if you are using it in a when expression:

when (something) {
    is ThisType -> 0
    is ThatType -> 42
}

You are sure covered all the cases. In a sense, they work like an enum, but more extensible.

6

With that in mind, why would you need enums in the language?

7

Well, I suppose enums are a lighter weight solution. And they map to Java enums, which you need to be able to represent in Kotlin.

8

I use sealed classes for events that might might contain varying kinds of data, which works really nice with kotlin’s smart casting capabilities.

e.g.

sealed class ScreenEvent
data class ItemSelected(val item: Item) : ScreenEvent()
data class TextEntered(val text: String) : ScreenEvent()
object BackButtonClick : ScreenEvent()

fun handleEvent(event: ScreenEvent) = when(event) {
    is ItemSelected -> doSomethingWithItem(event.item)
    is TextEntered -> doSomethingWithText(event.text)
    BackButtonClick -> navigateBack()
}

You can’t easily do that with standard enums

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