Stage 1 · Lesson 5 of 9
Sealed classes and when
A or sealed interface has a fixed set of subtypes that the compiler knows about, such as Loading, Success(data) and Error(message). A when expression over it must cover every subtype. So if you add a new state, the compiler points at each place you need to update.
Use an enum class when the options carry no data of their own, such as a sort order. Use a sealed type when each case holds its own fields. This is the standard way to model UI state in Android apps, and you will use it in the next stages.
Common trapa with isLoading, data and error fields that allows impossible combinations.
A question to check yourself, an AI practice session and 1 cheat sheet entry come with the full roadmap. Unlock
Resources for this stage
Start with· follow one of these
- GuideFreeKotlin Documentation
JetBrains
Start with the Kotlin tour, then the null safety and coroutines basics pages
- CourseFreeAndroid Basics with Compose
Google · Beginner
The Kotlin pathways in Units 1 to 3 teach the language with practice sets