Junior Mobile Developer (Android) Interview Questions: Complete Guide

Introduction

Android development powers billions of devices worldwide, making it one of the most important mobile platforms. With Kotlin as the preferred language and modern tools like Jetpack Compose, Android developers create rich, performant applications for diverse users.

This guide covers essential interview questions for Junior Android Developers. We explore Kotlin fundamentals, Android components, UI development, data persistence, networking, and best practices to help you prepare for your first Android developer role.

Kotlin Fundamentals (6 Questions)

1. What is the difference between `val` and `var` in Kotlin?

Answer:

val: Declares an immutable (read-only) variable. Once assigned, its value cannot be changed.
var: Declares a mutable variable. Its value can be changed after initialization.
Best Practice: Use val by default for safety. Only use var when you need to reassign the value.

val name = "John"  // Cannot be changed
var age = 25       // Can be changed
age = 26           // Valid
// name = "Jane"   // Error: Val cannot be reassigned

Rarity: Very Common Difficulty: Easy

2. Explain Nullable types and the Safe Call operator in Kotlin.

Answer: Kotlin's type system distinguishes between nullable and non-nullable types to prevent null pointer exceptions.

Nullable Type: Add ? after the type: String?
Safe Call (?.): Calls a method only if the object is not null
Elvis Operator (?:): Provides a default value if null
Not-null Assertion (!!): Throws exception if null (use sparingly)

var email: String? = "user@example.com"

// Safe call
val length = email?.length  // Returns null if email is null

// Elvis operator
val displayEmail = email ?: "No email"

// Let function
email?.let {
    println("Email: $it")
}

// Not-null assertion (dangerous)
val len = email!!.length  // Throws exception if email is null

Rarity: Very Common Difficulty: Easy

3. What is the difference between a `class` and a `data class` in Kotlin?

Answer:

Regular Class: Standard class definition
Data Class: Automatically generates useful methods for holding data
- equals() and hashCode()
- toString()
- copy() function
- componentN() functions for destructuring

// Regular class
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() generated)
println(user1)           // User(name=John, age=25) (toString() generated)

// Copy with modifications
val user3 = user1.copy(age = 26)

// Destructuring
val (name, age) = user1

Rarity: Very Common Difficulty: Easy

4. What are Lambda expressions and Higher-order functions in Kotlin?

Answer:

Lambda: Anonymous function that can be passed as a value
Higher-order Function: Function that takes functions as parameters or returns a function

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

// Higher-order function
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 operations
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]

Rarity: Very Common Difficulty: Medium

5. Explain Extension Functions in Kotlin.

Answer: Extension functions allow you to add new functions to existing classes without modifying their source code.

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

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

// Extension function with parameters
fun Int.times(action: () -> Unit) {
    repeat(this) {
        action()
    }
}

3.times {
    println("Hello")
}
// Prints "Hello" three times

Rarity: Common Difficulty: Easy

6. What is the difference between `==` and `===` in Kotlin?

Answer:

==: Structural equality (compares values using equals())
===: Referential equality (compares memory references)

val str1 = "Hello"
val str2 = "Hello"
val str3 = str1

println(str1 == str2)   // true (same value)
println(str1 === str2)  // true (string pool optimization)
println(str1 === str3)  // true (same reference)

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

println(list1 == list2)   // true (same content)
println(list1 === list2)  // false (different objects)

Rarity: Common Difficulty: Easy

Android Components (5 Questions)

7. What is an Activity and explain its lifecycle.

Answer: An Activity represents a single screen with a user interface. It has a well-defined lifecycle:

Loading diagram...

onCreate(): Activity is created. Initialize UI, set content view.
onStart(): Activity becomes visible.
onResume(): Activity is in foreground and interactive.
onPause(): Activity losing focus (another activity coming to foreground).
onStop(): Activity no longer visible.
onDestroy(): Activity is destroyed.

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        // Initialize views, setup data
    }
    
    override fun onStart() {
        super.onStart()
        // Activity becoming visible
    }
    
    override fun onResume() {
        super.onResume()
        // Activity is interactive, start animations
    }
    
    override fun onPause() {
        super.onPause()
        // Activity losing focus, pause ongoing actions
    }
}

Rarity: Very Common Difficulty: Easy

8. What is the difference between an Activity and a Fragment?

Answer:

Activity: Represents a full screen. Entry point for user interaction. Has its own lifecycle.
Fragment: Reusable portion of UI within an Activity. Multiple fragments can exist in one activity. Has its own lifecycle tied to the host activity.
Benefits of Fragments:
- Reusability across activities
- Modular UI components
- Support for tablets (multi-pane layouts)
- Navigation component integration

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)
        // Initialize 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()
    }
}

Rarity: Very Common Difficulty: Easy

9. What is an Intent and what are its types?

Answer: Intent is a messaging object used to request an action from another app component.

Explicit Intent: Specifies the exact component to start (by class name)
Implicit Intent: Declares a general action, and the system finds the appropriate component

// Explicit Intent - start specific activity
val intent = Intent(this, DetailActivity::class.java)
intent.putExtra("USER_ID", 123)
startActivity(intent)

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

// Implicit Intent - share text
val shareIntent = Intent(Intent.ACTION_SEND).apply {
    type = "text/plain"
    putExtra(Intent.EXTRA_TEXT, "Check this out!")
}
startActivity(Intent.createChooser(shareIntent, "Share via"))

// Implicit Intent - make phone call
val callIntent = Intent(Intent.ACTION_DIAL).apply {
    data = Uri.parse("tel:1234567890")
}
startActivity(callIntent)

Rarity: Very Common Difficulty: Easy

10. What is a Service in Android?

Answer: A Service is a component that runs in the background to perform long-running operations without a user interface.

Types:
- Foreground Service: Performs noticeable operations (music player). Shows notification.
- Background Service: Performs operations not directly noticed by user.
- Bound Service: Allows components to bind to it and interact.

class MyService : Service() {
    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
        // Perform background work
        Thread {
            // Long-running operation
            Thread.sleep(5000)
            stopSelf()
        }.start()
        
        return START_STICKY
    }
    
    override fun onBind(intent: Intent?): IBinder? {
        return null
    }
}

// Start service
val serviceIntent = Intent(this, MyService::class.java)
startService(serviceIntent)

// Stop service
stopService(serviceIntent)

Rarity: Common Difficulty: Medium

11. What is a BroadcastReceiver?

Answer: BroadcastReceiver is a component that responds to system-wide broadcast announcements.

Use Cases: Battery low, network connectivity changes, SMS received, boot completed
Registration:
- Static: In AndroidManifest.xml (limited in newer Android versions)
- Dynamic: In code (preferred)

class MyBroadcastReceiver : BroadcastReceiver() {
    override fun onReceive(context: Context?, intent: Intent?) {
        when (intent?.action) {
            Intent.ACTION_BATTERY_LOW -> {
                // Handle low battery
            }
            ConnectivityManager.CONNECTIVITY_ACTION -> {
                // Handle network change
            }
        }
    }
}

// Register dynamically
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)
    }
}

Rarity: Common Difficulty: Easy

UI Development (4 Questions)

12. What is the difference between LinearLayout, RelativeLayout, and ConstraintLayout?

Answer:

LinearLayout: Arranges children in a single row or column. Simple but can lead to nested layouts.
RelativeLayout: Positions children relative to each other or parent. More flexible but complex.
ConstraintLayout: Modern, flexible layout. Flat view hierarchy. Recommended for complex UIs.

<!-- ConstraintLayout example -->
<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="Title"
        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="Click Me"
        app:layout_constraintTop_toBottomOf="@id/title"
        app:layout_constraintStart_toStartOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Rarity: Very Common Difficulty: Easy

13. What is RecyclerView and how does it work?

Answer: RecyclerView is an efficient widget for displaying large lists by recycling views.

Components:
- Adapter: Binds data to views
- ViewHolder: Holds references to views (avoids findViewById calls)
- LayoutManager: Positions items (Linear, Grid, Staggered)

// 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)

Rarity: Very Common Difficulty: Medium

14. What is Jetpack Compose?

Answer: Jetpack Compose is Android's modern declarative UI toolkit.

Declarative: Describe what the UI should look like, not how to build it
Benefits: Less code, intuitive, powerful, accelerates development
Composable Functions: Building blocks of Compose UI

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

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

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

Rarity: Very Common Difficulty: Easy

15. What is the difference between `match_parent` and `wrap_content`?

Answer: These are layout parameters that define view dimensions:

match_parent: View expands to fill the parent's size
wrap_content: View sizes itself to fit its content
Fixed Size: Specific dp value (e.g., 100dp)

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
    
    <!-- Fills parent width, wraps content height -->
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Title" />
    
    <!-- Wraps both dimensions -->
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Click Me" />
    
    <!-- Fixed size -->
    <ImageView
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:src="@drawable/icon" />
</LinearLayout>

Rarity: Very Common Difficulty: Easy

Data & Networking (5 Questions)

16. How do you make a network request in Android?

Answer: Use libraries like Retrofit or OkHttp for networking. Avoid using HttpURLConnection directly.

// 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)
    }
}

// Usage in ViewModel
class UserViewModel : ViewModel() {
    fun fetchUser(userId: Int) {
        viewModelScope.launch {
            try {
                val user = RetrofitClient.apiService.getUser(userId)
                // Update UI
            } catch (e: Exception) {
                // Handle error
            }
        }
    }
}

Rarity: Very Common Difficulty: Medium

17. What is Room and how do you use it?

Answer: Room is an abstraction layer over SQLite for easier database access.

Components:
- Entity: Represents a table
- DAO (Data Access Object): Defines database operations
- Database: Database holder

// 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
            }
        }
    }
}

Rarity: Very Common Difficulty: Medium

18. What is SharedPreferences?

Answer: SharedPreferences stores small amounts of primitive data as key-value pairs.

Use Cases: User settings, preferences, simple flags
Not for: Large data, complex objects (use Room instead)

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

// Read data
val username = sharedPref.getString("username", "Guest")
val age = sharedPref.getInt("age", 0)
val isLoggedIn = sharedPref.getBoolean("isLoggedIn", false)

// Remove data
with(sharedPref.edit()) {
    remove("username")
    apply()
}

// Clear all
with(sharedPref.edit()) {
    clear()
    apply()
}

Rarity: Very Common Difficulty: Easy

19. What is the difference between `apply()` and `commit()` in SharedPreferences?

Answer: Both save changes to SharedPreferences, but differ in behavior:

apply(): Asynchronous. Returns immediately. Changes written to disk in background. No return value.
commit(): Synchronous. Blocks until changes are written. Returns boolean (success/failure).
Best Practice: Use apply() unless you need the return value.

val editor = sharedPref.edit()

// apply() - asynchronous, preferred
editor.putString("key", "value")
editor.apply()  // Returns immediately

// commit() - synchronous
editor.putString("key", "value")
val success = editor.commit()  // Waits for write, returns boolean
if (success) {
    // Save successful
}

Rarity: Common Difficulty: Easy

20. What is Coroutines in Kotlin?

Answer: Coroutines provide a way to write asynchronous code sequentially, making it easier to read and maintain.

Benefits: Lightweight, structured concurrency, exception handling
Key Concepts:
- suspend: Function that can be paused and resumed
- launch: Starts a coroutine (fire and forget)
- async: Starts a coroutine and returns a result
- Dispatchers: Control which thread the coroutine runs on

class MyViewModel : ViewModel() {
    fun loadData() {
        viewModelScope.launch {
            try {
                // Main thread
                showLoading()
                
                // Switch to IO thread for network call
                val data = withContext(Dispatchers.IO) {
                    fetchDataFromNetwork()
                }
                
                // Back to main thread
                updateUI(data)
            } catch (e: Exception) {
                handleError(e)
            }
        }
    }
    
    suspend fun fetchDataFromNetwork(): String {
        delay(2000)  // Simulates network delay
        return "Data from server"
    }
}

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

Rarity: Very Common Difficulty: Medium

Junior Mobile Developer (Android) Interview Questions: Complete Guide

Introduction

Kotlin Fundamentals (6 Questions)

1. What is the difference between `val` and `var` in Kotlin?

2. Explain Nullable types and the Safe Call operator in Kotlin.

3. What is the difference between a `class` and a `data class` in Kotlin?

4. What are Lambda expressions and Higher-order functions in Kotlin?

5. Explain Extension Functions in Kotlin.

6. What is the difference between `==` and `===` in Kotlin?

Android Components (5 Questions)

7. What is an Activity and explain its lifecycle.

8. What is the difference between an Activity and a Fragment?

9. What is an Intent and what are its types?

10. What is a Service in Android?

11. What is a BroadcastReceiver?

UI Development (4 Questions)

12. What is the difference between LinearLayout, RelativeLayout, and ConstraintLayout?

13. What is RecyclerView and how does it work?

14. What is Jetpack Compose?

15. What is the difference between `match_parent` and `wrap_content`?

Data & Networking (5 Questions)

16. How do you make a network request in Android?

17. What is Room and how do you use it?

18. What is SharedPreferences?

19. What is the difference between `apply()` and `commit()` in SharedPreferences?

20. What is Coroutines in Kotlin?

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Introduction

Kotlin Fundamentals (6 Questions)

1. What is the difference between val and var in Kotlin?

2. Explain Nullable types and the Safe Call operator in Kotlin.

3. What is the difference between a class and a data class in Kotlin?

4. What are Lambda expressions and Higher-order functions in Kotlin?

5. Explain Extension Functions in Kotlin.

6. What is the difference between == and === in Kotlin?

Android Components (5 Questions)

7. What is an Activity and explain its lifecycle.

8. What is the difference between an Activity and a Fragment?

9. What is an Intent and what are its types?

10. What is a Service in Android?

11. What is a BroadcastReceiver?

UI Development (4 Questions)

12. What is the difference between LinearLayout, RelativeLayout, and ConstraintLayout?

13. What is RecyclerView and how does it work?

14. What is Jetpack Compose?

15. What is the difference between match_parent and wrap_content?

Data & Networking (5 Questions)

16. How do you make a network request in Android?

17. What is Room and how do you use it?

18. What is SharedPreferences?

19. What is the difference between apply() and commit() in SharedPreferences?

20. What is Coroutines in Kotlin?

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Weekly career tips that actually work

1. What is the difference between `val` and `var` in Kotlin?

3. What is the difference between a `class` and a `data class` in Kotlin?

6. What is the difference between `==` and `===` in Kotlin?

15. What is the difference between `match_parent` and `wrap_content`?

19. What is the difference between `apply()` and `commit()` in SharedPreferences?