Generics, Sealed Classes & Flow

Generics

class Box<T>(val value: T)
val intBox = Box(42)
val strBox = Box("hi")

fun <T> firstOrNull(list: List<T>): T? = if (list.isEmpty()) null else list[0]

Sealed classes (great for state)

sealed class Result {
    data class Success(val data: String) : Result()
    data class Error(val message: String) : Result()
    object Loading : Result()
}

fun render(r: Result) = when (r) {
    is Result.Success -> show(r.data)
    is Result.Error   -> showError(r.message)
    Result.Loading    -> showSpinner()
    // no else needed: compiler knows all cases
}

Flow (streams of values)

fun ticker(): Flow<Int> = flow {
    var i = 0
    while (true) { emit(i++); delay(1000) }
}

lifecycleScope.launch {
    ticker().collect { println(it) }   // 0, 1, 2, ...
}

Use sealed classes to model UI state (Loading/Success/Error) — combined with Flow they make reactive, bug-resistant apps.