Preguntas para Entrevistas de Desarrollador Mobile Junior (Android): Guía Completa

val name = "John"  // No se puede cambiar
var age = 25       // Se puede cambiar
age = 26           // Válido
// name = "Jane"   // Error: Val no se puede reasignar

var email: String? = "[email protected]"

// Safe call
val length = email?.length  // Devuelve null si email es nulo

// Operador Elvis
val displayEmail = email ?: "Sin correo electrónico"

// Función Let
email?.let {
    println("Correo electrónico: $it")
}

// Not-null assertion (peligroso)
val len = email!!.length  // Lanza una excepción si email es nulo

// Clase regular
class Person(val name: String, val age: Int)

// Data class
data class User(val name: String, val age: Int)

val user1 = User("John", 25)
val user2 = User("John", 25)

println(user1 == user2)  // true (equals() generado)
println(user1)           // User(name=John, age=25) (toString() generado)

// Copiar con modificaciones
val user3 = user1.copy(age = 26)

// Desestructuración
val (name, age) = user1

// Expresión Lambda
val sum = { a: Int, b: Int -> a + b }
println(sum(5, 3))  // 8

// Función de orden superior
fun calculate(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)
}

val result = calculate(10, 5) { x, y -> x * y }
println(result)  // 50

// Operaciones de colección
val numbers = listOf(1, 2, 3, 4, 5)
val doubled = numbers.map { it * 2 }
val evens = numbers.filter { it % 2 == 0 }

println(doubled)  // [2, 4, 6, 8, 10]
println(evens)    // [2, 4]

// Función de extensión para String
fun String.isPalindrome(): Boolean {
    return this == this.reversed()
}

println("radar".isPalindrome())  // true
println("hello".isPalindrome())  // false

// Función de extensión con parámetros
fun Int.times(action: () -> Unit) {
    repeat(this) {
        action()
    }
}

3.times {
    println("Hola")
}
// Imprime "Hola" tres veces

val str1 = "Hola"
val str2 = "Hola"
val str3 = str1

println(str1 == str2)   // true (mismo valor)
println(str1 === str2)  // true (optimización del string pool)
println(str1 === str3)  // true (misma referencia)

val list1 = listOf(1, 2, 3)
val list2 = listOf(1, 2, 3)

println(list1 == list2)   // true (mismo contenido)
println(list1 === list2)  // false (diferentes objetos)

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        // Inicializar vistas, configurar datos
    }
    
    override fun onStart() {
        super.onStart()
        // La Activity se vuelve visible
    }
    
    override fun onResume() {
        super.onResume()
        // La Activity es interactiva, iniciar animaciones
    }
    
    override fun onPause() {
        super.onPause()
        // La Activity pierde el foco, pausar acciones en curso
    }
}

class MyFragment : Fragment() {
    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_my, container, false)
    }
    
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        // Inicializar vistas
    }
}

// En la Activity
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        
        supportFragmentManager.beginTransaction()
            .replace(R.id.fragment_container, MyFragment())
            .commit()
    }
}

// Intent Explícito - iniciar una Activity específica
val intent = Intent(this, DetailActivity::class.java)
intent.putExtra("USER_ID", 123)
startActivity(intent)

// Intent Implícito - abrir URL
val webIntent = Intent(Intent.ACTION_VIEW, Uri.parse("https://ejemplo.com"))
startActivity(webIntent)

// Intent Implícito - compartir texto
val shareIntent = Intent(Intent.ACTION_SEND).apply {
    type = "text/plain"
    putExtra(Intent.EXTRA_TEXT, "¡Mira esto!")
}
startActivity(Intent.createChooser(shareIntent, "Compartir vía"))

// Intent Implícito - hacer una llamada telefónica
val callIntent = Intent(Intent.ACTION_DIAL).apply {
    data = Uri.parse("tel:1234567890")
}
startActivity(callIntent)

class MyService : Service() {
    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
        // Realizar trabajo en segundo plano
        Thread {
            // Operación de larga duración
            Thread.sleep(5000)
            stopSelf()
        }.start()
        
        return START_STICKY
    }
    
    override fun onBind(intent: Intent?): IBinder? {
        return null
    }
}

// Iniciar servicio
val serviceIntent = Intent(this, MyService::class.java)
startService(serviceIntent)

// Detener servicio
stopService(serviceIntent)

class MyBroadcastReceiver : BroadcastReceiver() {
    override fun onReceive(context: Context?, intent: Intent?) {
        when (intent?.action) {
            Intent.ACTION_BATTERY_LOW -> {
                // Manejar batería baja
            }
            ConnectivityManager.CONNECTIVITY_ACTION -> {
                // Manejar cambio de red
            }
        }
    }
}

// Registrar dinámicamente
class MainActivity : AppCompatActivity() {
    private val receiver = MyBroadcastReceiver()
    
    override fun onStart() {
        super.onStart()
        val filter = IntentFilter(Intent.ACTION_BATTERY_LOW)
        registerReceiver(receiver, filter)
    }
    
    override fun onStop() {
        super.onStop()
        unregisterReceiver(receiver)
    }
}

<!-- Ejemplo de ConstraintLayout -->
<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    
    <TextView
        android:id="@+id/title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Título"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
    
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Haz clic aquí"
        app:layout_constraintTop_toBottomOf="@id/title"
        app:layout_constraintStart_toStartOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

// Data class
data class User(val name: String, val email: String)

// ViewHolder
class UserViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
    private val nameText: TextView = itemView.findViewById(R.id.nameText)
    private val emailText: TextView = itemView.findViewById(R.id.emailText)
    
    fun bind(user: User) {
        nameText.text = user.name
        emailText.text = user.email
    }
}

// Adapter
class UserAdapter(private val users: List<User>) : 
    RecyclerView.Adapter<UserViewHolder>() {
    
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.item_user, parent, false)
        return UserViewHolder(view)
    }
    
    override fun onBindViewHolder(holder: UserViewHolder, position: Int) {
        holder.bind(users[position])
    }
    
    override fun getItemCount() = users.size
}

// En la Activity
recyclerView.layoutManager = LinearLayoutManager(this)
recyclerView.adapter = UserAdapter(userList)

@Composable
fun Greeting(name: String) {
    Text(text = "¡Hola, $name!")
}

@Composable
fun Counter() {
    var count by remember { mutableStateOf(0) }
    
    Column(
        modifier = Modifier.padding(16.dp),
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        Text(text = "Conteo: $count")
        Button(onClick = { count++ }) {
            Text("Incrementar")
        }
    }
}

@Preview
@Composable
fun PreviewCounter() {
    Counter()
}

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
    
    <!-- Llena el ancho del padre, ajusta la altura al contenido -->
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Título" />
    
    <!-- Ajusta ambas dimensiones -->
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Haz clic aquí" />
    
    <!-- Tamaño fijo -->
    <ImageView
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:src="@drawable/icon" />
</LinearLayout>

// Configuración de Retrofit
interface ApiService {
    @GET("users/{id}")
    suspend fun getUser(@Path("id") userId: Int): User
    
    @POST("users")
    suspend fun createUser(@Body user: User): User
}

object RetrofitClient {
    private const val BASE_URL = "https://api.ejemplo.com/"
    
    val apiService: ApiService by lazy {
        Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(ApiService::class.java)
    }
}

// Uso en ViewModel
class UserViewModel : ViewModel() {
    fun fetchUser(userId: Int) {
        viewModelScope.launch {
            try {
                val user = RetrofitClient.apiService.getUser(userId)
                // Actualizar UI
            } catch (e: Exception) {
                // Manejar error
            }
        }
    }
}

// Entity
@Entity(tableName = "users")
data class User(
    @PrimaryKey(autoGenerate = true) val id: Int = 0,
    @ColumnInfo(name = "name") val name: String,
    @ColumnInfo(name = "email") val email: String
)

// DAO
@Dao
interface UserDao {
    @Query("SELECT * FROM users")
    fun getAllUsers(): List<User>
    
    @Query("SELECT * FROM users WHERE id = :userId")
    fun getUserById(userId: Int): User?
    
    @Insert
    suspend fun insertUser(user: User)
    
    @Delete
    suspend fun deleteUser(user: User)
    
    @Update
    suspend fun updateUser(user: User)
}

// Database
@Database(entities = [User::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun userDao(): UserDao
    
    companion object {
        @Volatile
        private var INSTANCE: AppDatabase? = null
        
        fun getDatabase(context: Context): AppDatabase {
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "app_database"
                ).build()
                INSTANCE = instance
                instance
            }
        }
    }
}

// Guardar datos
val sharedPref = getSharedPreferences("MisPrefs", Context.MODE_PRIVATE)
with(sharedPref.edit()) {
    putString("username", "John")
    putInt("age", 25)
    putBoolean("isLoggedIn", true)
    apply()  // o commit()
}

// Leer datos
val username = sharedPref.getString("username", "Invitado")
val age = sharedPref.getInt("age", 0)
val isLoggedIn = sharedPref.getBoolean("isLoggedIn", false)

// Eliminar datos
with(sharedPref.edit()) {
    remove("username")
    apply()
}

// Limpiar todo
with(sharedPref.edit()) {
    clear()
    apply()
}

val editor = sharedPref.edit()

// apply() - asíncrono, preferido
editor.putString("key", "value")
editor.apply()  // Regresa inmediatamente

// commit() - síncrono
editor.putString("key", "value")
val success = editor.commit()  // Espera la escritura, devuelve un booleano
if (success) {
    // Guardado exitoso
}

class MyViewModel : ViewModel() {
    fun loadData() {
        viewModelScope.launch {
            try {
                // Hilo principal
                showLoading()
                
                // Cambiar al hilo de IO para la llamada de red
                val data = withContext(Dispatchers.IO) {
                    fetchDataFromNetwork()
                }
                
                // De vuelta al hilo principal
                updateUI(data)
            } catch (e: Exception) {
                handleError(e)
            }
        }
    }
    
    suspend fun fetchDataFromNetwork(): String {
        delay(2000)  // Simula el retraso de la red
        return "Datos del servidor"
    }
}

// Ejecución paralela
viewModelScope.launch {
    val deferred1 = async { fetchUser(1) }
    val deferred2 = async { fetchUser(2) }
    
    val user1 = deferred1.await()
    val user2 = deferred2.await()
}