Top 20 Kotlin interview questions and answers 2025-2026

1️⃣ What are the key features of Kotlin?

Answer:

• Concise and expressive syntax
• Null safety (? and !!)
• Interoperable with Java
• Extension functions
• Coroutines for asynchronous programming
• Data classes and Sealed classes

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2️⃣ Difference between val and var.

Answer:

• val → immutable (like final in Java)
• var → mutable, value can change

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3️⃣ What are Data Classes in Kotlin?

Answer:

• Classes used to hold data
• Auto-generates: toString(), equals(), hashCode(), copy(), componentN()

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

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4️⃣ Explain Null Safety in Kotlin.

Answer:

• Kotlin prevents NullPointerException using ? operator
• Safe call: user?.name
• Elvis operator: val length = user?.name?.length ?: 0
• Not-null assertion: user!!.name (throws exception if null)

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5️⃣ Difference between == and ===.

Answer:

• == → structural equality (checks values)
• === → referential equality (checks if objects point to same memory)

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6️⃣ What are Extension Functions?

Answer:

• Add functions to existing classes without inheritance

fun String.addExclamation() = this + "!"
val text = "Hello".addExclamation()  // "Hello!"

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7️⃣ Explain Sealed Classes.

Answer:

• Used for restricted class hierarchies
• All subclasses known at compile time
• Useful in when expressions

sealed class Result
data class Success(val data: String) : Result()
data class Error(val error: String) : Result()

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8️⃣ Difference between open, abstract, and final classes.

Answer:

• final → default, cannot inherit
• open → can be inherited
• abstract → cannot be instantiated, may have abstract members

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9️⃣ What are Coroutines in Kotlin?

Answer:

• Lightweight threads for asynchronous programming
• Use suspend functions, launch, async, and withContext
• Reduces callback hell and improves performance

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10️⃣ Difference between launch and async.

Answer:

• launch → returns Job, does not return result
• async → returns Deferred<T>, can return result using await()

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11️⃣ Explain lateinit vs lazy.

Answer:

• lateinit → for var, initialized later, avoids null
• lazy → for val, initialized on first access

lateinit var name: String
val age: Int by lazy { 25 }

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12️⃣ Difference between object and companion object.

Answer:

• object → singleton instance
• companion object → static members in class

class MyClass {
    companion object { val id = 1 }
}

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13️⃣ Explain Higher-Order Functions.

Answer:

• Functions that take functions as parameters or return functions

fun operate(x: Int, y: Int, op: (Int, Int) -> Int) = op(x, y)
val sum = operate(2, 3) { a, b -> a + b }  // 5

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14️⃣ Difference between map, flatMap, filter in Kotlin.

Answer:

• map → transforms each element
• flatMap → maps and flattens nested collections
• filter → selects elements based on condition

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15️⃣ What are Inline Functions?

Answer:

• Functions marked inline are inlined at call site
• Reduces runtime overhead, useful for higher-order functions

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16️⃣ Explain typealias in Kotlin.

Answer:

• Provides alternative name for type
• Useful for long generic types

typealias StringMap = Map<String, String>
val data: StringMap = mapOf("a" to "1")

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17️⃣ What are Kotlin Collections?

Answer:

• List → ordered, can be mutable/immutable
• Set → unique elements
• Map → key-value pairs
• Functions: map, filter, forEach, reduce, fold

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18️⃣ Difference between Any, Unit, and Nothing.

Answer:

• Any → supertype of all non-null types
• Unit → like void in Java
• Nothing → type with no value, useful for exceptions

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19️⃣ Explain Kotlin Delegation.

Answer:

• Kotlin supports delegating responsibilities to another object
• by keyword used

interface Base { fun print() }
class BaseImpl(val x: Int) : Base { override fun print() = println(x) }
class Derived(b: Base) : Base by b

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20️⃣ Difference between const val and val.

Answer:

• val → read-only runtime constant
• const val → compile-time constant, can only hold primitive types or String

Happy Learning..