Junior-Android-Entwickler: Interviewfragen und Antworten

val name = "John"  // Kann nicht geändert werden
var age = 25       // Kann geändert werden
age = 26           // Gültig
// name = "Jane"   // Fehler: Val kann nicht neu zugewiesen werden

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

// Safe Call
val length = email?.length  // Gibt null zurück, wenn email null ist

// Elvis-Operator
val displayEmail = email ?: "Keine E-Mail"

// Let-Funktion
email?.let {
    println("E-Mail: $it")
}

// Not-null Assertion (gefährlich)
val len = email!!.length  // Wirft eine Ausnahme, wenn email null ist

// Normale Klasse
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() generiert)
println(user1)           // User(name=John, age=25) (toString() generiert)

// Kopieren mit Änderungen
val user3 = user1.copy(age = 26)

// Destrukturierung
val (name, age) = user1

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

// Higher-Order-Funktion
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

// Collection-Operationen
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]

// Extension Function für String
fun String.isPalindrome(): Boolean {
    return this == this.reversed()
}

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

// Extension Function mit Parametern
fun Int.times(action: () -> Unit) {
    repeat(this) {
        action()
    }
}

3.times {
    println("Hallo")
}
// Gibt "Hallo" dreimal aus

val str1 = "Hallo"
val str2 = "Hallo"
val str3 = str1

println(str1 == str2)   // true (gleicher Wert)
println(str1 === str2)  // true (String-Pool-Optimierung)
println(str1 === str3)  // true (gleiche Referenz)

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

println(list1 == list2)   // true (gleicher Inhalt)
println(list1 === list2)  // false (verschiedene Objekte)

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        // Initialisieren Sie Views, richten Sie Daten ein
    }
    
    override fun onStart() {
        super.onStart()
        // Activity wird sichtbar
    }
    
    override fun onResume() {
        super.onResume()
        // Activity ist interaktiv, starten Sie Animationen
    }
    
    override fun onPause() {
        super.onPause()
        // Activity verliert den Fokus, pausieren Sie laufende Aktionen
    }
}

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)
        // Initialisieren Sie Views
    }
}

// In 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()
    }
}

// Expliziter Intent - starten Sie eine bestimmte Activity
val intent = Intent(this, DetailActivity::class.java)
intent.putExtra("USER_ID", 123)
startActivity(intent)

// Impliziter Intent - URL öffnen
val webIntent = Intent(Intent.ACTION_VIEW, Uri.parse("https://example.com"))
startActivity(webIntent)

// Impliziter Intent - Text teilen
val shareIntent = Intent(Intent.ACTION_SEND).apply {
    type = "text/plain"
    putExtra(Intent.EXTRA_TEXT, "Schau dir das an!")
}
startActivity(Intent.createChooser(shareIntent, "Teilen via"))

// Impliziter Intent - Telefonanruf tätigen
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 {
        // Führen Sie Hintergrundarbeiten aus
        Thread {
            // Langwierige Operation
            Thread.sleep(5000)
            stopSelf()
        }.start()
        
        return START_STICKY
    }
    
    override fun onBind(intent: Intent?): IBinder? {
        return null
    }
}

// Service starten
val serviceIntent = Intent(this, MyService::class.java)
startService(serviceIntent)

// Service stoppen
stopService(serviceIntent)

class MyBroadcastReceiver : BroadcastReceiver() {
    override fun onReceive(context: Context?, intent: Intent?) {
        when (intent?.action) {
            Intent.ACTION_BATTERY_LOW -> {
                // Behandeln Sie niedrigen Batteriestand
            }
            ConnectivityManager.CONNECTIVITY_ACTION -> {
                // Behandeln Sie Netzwerkänderungen
            }
        }
    }
}

// Dynamisch registrieren
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)
    }
}

<!-- ConstraintLayout Beispiel -->
<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="Titel"
        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="Klick mich"
        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
}

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

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

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

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

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
    
    <!-- Füllt die Elternbreite aus, umschließt die Inhaltshöhe -->
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Titel" />
    
    <!-- Umschließt beide Dimensionen -->
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Klick mich" />
    
    <!-- Feste Größe -->
    <ImageView
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:src="@drawable/icon" />
</LinearLayout>

// Retrofit-Setup
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.example.com/"
    
    val apiService: ApiService by lazy {
        Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(ApiService::class.java)
    }
}

// Verwendung in ViewModel
class UserViewModel : ViewModel() {
    fun fetchUser(userId: Int) {
        viewModelScope.launch {
            try {
                val user = RetrofitClient.apiService.getUser(userId)
                // UI aktualisieren
            } catch (e: Exception) {
                // Fehler behandeln
            }
        }
    }
}

// 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
            }
        }
    }
}

// Daten speichern
val sharedPref = getSharedPreferences("MyPrefs", Context.MODE_PRIVATE)
with(sharedPref.edit()) {
    putString("username", "John")
    putInt("age", 25)
    putBoolean("isLoggedIn", true)
    apply()  // oder commit()
}

// Daten lesen
val username = sharedPref.getString("username", "Gast")
val age = sharedPref.getInt("age", 0)
val isLoggedIn = sharedPref.getBoolean("isLoggedIn", false)

// Daten entfernen
with(sharedPref.edit()) {
    remove("username")
    apply()
}

// Alles löschen
with(sharedPref.edit()) {
    clear()
    apply()
}

val editor = sharedPref.edit()

// apply() - asynchron, bevorzugt
editor.putString("key", "value")
editor.apply()  // Gibt sofort zurück

// commit() - synchron
editor.putString("key", "value")
val success = editor.commit()  // Wartet auf das Schreiben, gibt Boolean zurück
if (success) {
    // Speichern erfolgreich
}

class MyViewModel : ViewModel() {
    fun loadData() {
        viewModelScope.launch {
            try {
                // Hauptthread
                showLoading()
                
                // Wechseln Sie zum IO-Thread für den Netzwerkaufruf
                val data = withContext(Dispatchers.IO) {
                    fetchDataFromNetwork()
                }
                
                // Zurück zum Hauptthread
                updateUI(data)
            } catch (e: Exception) {
                handleError(e)
            }
        }
    }
    
    suspend fun fetchDataFromNetwork(): String {
        delay(2000)  // Simuliert Netzwerkverzögerung
        return "Daten vom Server"
    }
}

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