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Kotlin Interview Questions

1. What is Kotlin?

Kotlin is a modern, statically typed programming language that runs on the JVM, and is fully interoperable with Java. It's officially supported by Google for Android development.

2. How is Kotlin better than Java?

  • Null Safety
  • Less boilerplate code
  • Extension functions
  • Coroutines for async code
  • Data classes
  • Smart casting

3. What is a val vs var in Kotlin?

  • val: Immutable (like final in Java)
  • var: Mutable (value can change)
val name = "Sahand" // read-only
Var age = 30        // mutable

4. What is null safety in Kotlin?

Kotlin avoids NullPointerException by making types non-nullable by default.

var name: String = "Kotlin" 
name = null //  compile error  

var name2: String? = null //  Nullable

5. What is a data class in Kotlin?

Data classes automatically provide equals(), hashCode(), toString(), and copy() methods.

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

6. What are extension functions?

They let you add functions to existing classes without modifying them.

fun String.capitalizeFirst(): String {     
       return this.replaceFirstChar { it.uppercase() } 
}

7. What is a companion object?

Used to define static members in a class.

class MyClass {     
      companion object {         
           fun greet() = "Hello!"    
       } 
}

8. What are higher-order functions?

Functions that take functions as parameters or return functions.

fun operate(x: Int, y: Int, op: (Int, Int) -> Int): Int {     
        return op(x, y) 
}

9. What is the Elvis operator (?:) in Kotlin?

Used to provide a default value when an expression is null.

val name: String? = null 
Val displayName = name ?: "Guest"

10. What is safe call operator (?.) in Kotlin?

Used to safely access a property or call a function on a nullable object.

val length = name?.length

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

  • =: structural equality (like equals() in java)
  • ==: referential equality (compares memory reference)
12. What are coroutines in Kotlin?
Coroutines simplify asynchronous code. They're lightweight threads managed by Kotlin.
GlobalScope.launch {     
           delay(1000)     
           println("Hello from coroutine")
} 
13. What is a sealed class in Kotlin?
Restricts class inheritance to a limited set of types — good for representing state.
sealed class Result 
class Success(val data: String) : Result() 
Class Error(val message: String) : Result() 
14. What are default and named arguments?
fun greet(name: String = "Guest", message: String = "Hi") {     
             println("$message, $name!") 
}  

Greet()                                     // Hi, Guest! 
greet(message = "Hello")    // Hello, Guest! 
15. How to create a singleton in Kotlin?
Using the object keyword.
object AppConfig {     
     val version = "1.0"
} 
16. What is a lambda in Kotlin?
A lambda is an anonymous function.
val add = { x: Int, y: Int -> x + y }
17. What is a lateinit keyword?
Used to declare a non-null variable that will be initialized later.
lateinit var name: String
18. What is the use of is keyword in Kotlin?
Checks the type of an object.
if (user is String) {     
      println(user.length)
} 
19. What are ranges in Kotlin?
Used to define a range of values and iterate through them.
for (i in 1..5) { print(i) }       // 1 to 5 
For (i in 5 downTo 1 step 2) { }   // 5,3,1 
20. What is Destructuring Declaration?
Used to unpack a class into multiple variables.
data class User(val name: String, 
Val age: Int) val (n, a) = User("Sahand", 30)