Kotlin Idioms for Android

Kotlin smooths over many rough edges of the Android Java APIs. Here are the idioms you will use constantly.

Platform types and null safety

Values returned from Java/Android APIs have platform types — Kotlin does not know whether they are nullable. The compiler lets you treat them as either, but a wrong assumption crashes at runtime:

val extras = intent.extras           // platform type Bundle!
val name = extras?.getString("name") // be defensive: treat as nullable
    ?: "guest"

When calling Android/Java APIs, assume return values can be null unless the docs or annotations say otherwise. Use ?. and ?: at the boundary.

Scope functions for setup

apply and also make object configuration concise:

val intent = Intent(this, DetailActivity::class.java).apply {
    putExtra("id", 42)
    flags = Intent.FLAG_ACTIVITY_NEW_TASK
}

Concise click and callbacks

Lambdas replace anonymous inner classes:

button.setOnClickListener { viewModel.submit() }

In Compose this is even simpler — onClick = { ... }.

Extension functions for boilerplate

Add helpers to framework types:

fun Context.toast(message: String) =
    Toast.makeText(this, message, Toast.LENGTH_SHORT).show()

// usage inside an Activity/Context
toast("Saved")

`when` over sealed UI state

Model screen state as a sealed type and render exhaustively:

sealed interface UiState {
    data object Loading : UiState
    data class Success(val items: List<String>) : UiState
    data class Error(val message: String) : UiState
}

@Composable
fun Screen(state: UiState) = when (state) {
    UiState.Loading -> Spinner()
    is UiState.Success -> ItemList(state.items)
    is UiState.Error -> ErrorBanner(state.message)
}

This pattern — a sealed UiState rendered by an exhaustive when — is the backbone of the architecture in Part 8.

Exercises

Write a Context.toast(...) extension and use it.
Model a UiState sealed interface with Loading, Success, and Error, and write an exhaustive when over it.

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