Variables & Functions
val name = "fixed" // immutable
var count = 0 // mutable
val x: Int = 5 // explicit type
fun add(a: Int, b: Int): Int = a + b
fun greet(name: String = "there") = "Hi, $name"
// Lambda
val square: (Int) -> Int = { it * it }
Collections
val list = listOf(1, 2, 3) // immutable
val mutable = mutableListOf(1, 2)
val map = mapOf("a" to 1, "b" to 2)
list.map { it * 2 }
list.filter { it > 1 }
list.forEach { println(it) }
list.firstOrNull { it > 5 }
list.sortedBy { it }
Null Safety
var name: String? = null // nullable type
name?.length // safe call → null if name is null
name?.length ?: 0 // Elvis operator: default if null
name!!.length // force unwrap (crashes if null)
if (name != null) {
// smart-cast: name is String here
println(name.length)
}
Classes & Data Classes
data class User(val id: Int, val name: String)
// auto: equals(), hashCode(), toString(), copy()
val u = User(1, "Ana")
val u2 = u.copy(name = "Ben")
sealed interface Result
data class Ok(val value: String) : Result
data class Err(val msg: String) : Result
when (result) {
is Ok -> println(result.value)
is Err -> println(result.msg)
}
Coroutines
viewModelScope.launch {
val data = withContext(Dispatchers.IO) { api.getData() }
_uiState.value = data
}
suspend fun loadUser(id: Int): User = api.getUser(id)
// Flow
val flow: Flow<Int> = flowOf(1, 2, 3)
flow.collect { println(it) }
flow.stateIn(scope, SharingStarted.WhileSubscribed(5000), initial)
Compose State
var count by remember { mutableStateOf(0) }
val vmState by viewModel.uiState.collectAsState()
// Derived state (recomputes only when inputs change)
val isValid by remember(email) { derivedStateOf { email.contains("@") } }
// Side effect on composition
LaunchedEffect(key1 = userId) {
viewModel.load(userId)
}
Compose Layout
Column(Modifier.padding(16.dp)) { /* children stacked vertically */ }
Row(Modifier.fillMaxWidth()) { /* children in a row */ }
Box(Modifier.size(100.dp)) { /* stacked/overlapping */ }
Modifier
.fillMaxWidth()
.padding(12.dp)
.background(Color.White)
.clickable { /* handle tap */ }
Compose Lists
LazyColumn {
items(list, key = { it.id }) { item ->
Text(item.title)
}
}
LazyRow { items(images) { img -> AsyncImage(img) } }
Room
@Entity data class Note(@PrimaryKey(autoGenerate = true) val id: Int = 0, val title: String)
@Dao interface NoteDao {
@Query("SELECT * FROM Note") fun getAll(): Flow<List<Note>>
@Insert suspend fun insert(note: Note)
@Delete suspend fun delete(note: Note)
}
@Database(entities = [Note::class], version = 1)
abstract class AppDb : RoomDatabase() { abstract fun noteDao(): NoteDao }
