Complete kotlin interview questions and answers

Kotlin interviews typically cover the language's core features, its interoperability with Java, Android-specific topics (if applicable), and modern concepts like coroutines and Jetpack Compose. Preparation should address different experience levels, from foundational syntax to advanced architectural patterns. 

1. What is Kotlin?

Kotlin is a modern, statically typed programming language developed by JetBrains and officially supported by Google for Android.

2. Why use Kotlin over Java?

🔸 Less boilerplate code
🔸 Null safety
🔸 Better readability
🔸 Interoperable with Java

3. Is Kotlin statically typed?

Yes, Kotlin is statically typed, meaning types are checked at compile time.

4. What are Kotlin features?

🔸 Null safety
🔸 Extension functions
🔸 Data classes
🔸 Coroutines
🔸 Smart casts

5. What is val vs var?

🔸 val – Immutable (read-only)
🔸 var – Mutable

6. What is type inference?

Kotlin automatically infers variable types, reducing verbosity.

7. What is a data class?

Used to hold data and automatically provides:
🔸 equals()
🔸 hashCode()
🔸 toString()
🔸 copy()

8. What is a primary constructor?

Defined in the class header and initializes properties.

9. What is a secondary constructor?

Additional constructor used for extra initialization logic.

10. What is a companion object?

Used to define static-like members in Kotlin classes.

NULL SAFETY & CORE CONCEPTS

11. What is Null Safety in Kotlin?

Prevents NullPointerException at compile time.

12. What is nullable type?

🔸 Defined using ?
🔸 Example: String?

13. What is !! operator?

🔸 Forces non-null
🔸 Throws NPE if value is null

14. What is safe call operator (?.)?

🔸 Executes only if object is not null

15. What is Elvis operator (?:)?

🔸 Provides default value if null

16. What are smart casts?

Kotlin automatically casts types after null or type checks.

17. What is lateinit?

🔸 Used with var
🔸 Initialized later
🔸 Cannot be used with primitive types

18. What is lazy initialization?

🔸 Value initialized when first accessed
🔸 Thread-safe by default

FUNCTIONS & LAMBDAS

19. What is a higher-order function?

A function that:
🔸 Takes another function as parameter
🔸 Returns a function

20. What is a lambda expression?

Anonymous function used for functional programming.

21. What are inline functions?

🔸 Reduces overhead of lambda calls
🔸 Improves performance

22. What are default arguments?

Functions can have default parameter values.

23. What are named arguments?

Allows passing parameters using parameter names.

24. What is tail recursion?

Optimizes recursive calls to avoid stack overflow.

COLLECTIONS & EXTENSIONS

25. Mutable vs Immutable collections

🔸 Mutable – Can be modified
🔸 Immutable – Read-only

26. What are extension functions?

Add new functions to classes without modifying them.

27. What is map, filter, reduce?

🔸 map – Transform data
🔸 filter – Select data
🔸 reduce – Aggregate data

28. What is destructuring declaration?

Allows unpacking objects into variables.

29. What is Pair and Triple?

🔸 Holds 2 or 3 values
🔸 Lightweight data holders

30. What is sealed class?

🔸 Restricted class hierarchy
🔸 Used for state management

OBJECT-ORIENTED & ADVANCED KOTLIN

31. Difference between abstract class and interface

🔸 Abstract class can have state
🔸 Interface supports multiple inheritance

32. What is object declaration?

Creates a singleton instance.

33. What is object expression?

Creates anonymous objects.

34. What is delegation?

Class delegates responsibility to another class using by.

35. What is lateinit vs lazy?

🔸 lateinit – For vars, manual init
🔸 lazy – For vals, automatic init

36. What is inline class (value class)?

Wraps a single value with no runtime overhead.

37. What is reified keyword?

Allows access to generic type at runtime.

38. What is enum class?

Defines a fixed set of constants.

39. What is scope function?

Functions like:
🔸 let
🔸 run
🔸 apply
🔸 also
🔸 with

40. Difference between let, apply, also

🔸 let – Nullable handling
🔸 apply – Object configuration
🔸 also – Side effects

COROUTINES & CONCURRENCY (ADVANCED LEVEL)

41. What are coroutines?

Lightweight threads for asynchronous programming.

42. Coroutine vs Thread

🔸 Coroutines are lightweight
🔸 Better performance
🔸 Less memory

43. What is suspend function?

Can pause and resume execution without blocking thread.

44. What is CoroutineScope?

Defines the lifecycle of coroutines.

45. What is Dispatcher?

Determines which thread coroutine runs on.

46. Types of Dispatchers

🔸 Main
🔸 IO
🔸 Default

47. What is async vs launch?

🔸 launch – No return value
🔸 async – Returns Deferred result

48. What is structured concurrency?

Ensures coroutines are properly scoped and cancelled.

49. What is Flow?

Cold asynchronous data stream.

50. StateFlow vs SharedFlow

🔸 StateFlow – Holds state
🔸 SharedFlow – Event-based

Why Kotlin is preferred in modern Android development

🔸 Safer code
🔸 Better performance
🔸 Less boilerplate
🔸 Strong community support