Questions about sealded class, in the sample here <https://developer.android.com/codelabs/basic-andr...

# compose

[Pablo]

Questions about sealded class, in the sample here https://developer.android.com/codelabs/basic-android-kotlin-compose-getting-data-intern[…]elabs%2Fbasic-android-kotlin-compose-getting-data-internet they declare a sealed class:

sealed interface MarsUiState {
    data class Success(val photos: String) : MarsUiState
    object Error : MarsUiState
    object Loading : MarsUiState
}

why they use interface instead of class? why object for Error and Loading instead of class? object is a singleton.

[Stylianos Gakis]

I’d flip both your questions around.

Why sealed class instead of sealed interface?
Why class instead of object?

[Anselmo Alexandre]

I’d also say that you first need to understand the use-cases of sealed class, sealed interface and `object`s. Refer to this https://kotlinlang.org/docs/sealed-classes.html#declare-a-sealed-class-or-interface

[Zach Klippenstein (he/him) [MOD]]

I think the only reason to use a sealed class is:
1. you’re on an old version of kotlin that doesn’t support sealed interfaces (I’m so sorry)
2. You need the whole sealed type to subclass a class.

[Zach Klippenstein (he/him) [MOD]]

If you used a class for children that don’t have parameters, you’d still be allocating a new one every time you need one. But that’s wasteful, because all instances would be identical. So using an object keeps the allocations at 1.