Extensions resolved statically in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extension function call with base type reference

What is the output of this Kotlin code snippet?

Kotlin

open class Animal
class Dog : Animal()

fun Animal.sound() = "Animal sound"
fun Dog.sound() = "Dog sound"

fun main() {
    val animal: Animal = Dog()
    println(animal.sound())
}

A Compilation error

B Runtime exception

C Animal sound

D Dog sound

💡 Hint

Extension functions are resolved based on the declared type, not the runtime type.

In Kotlin, extension functions are resolved statically using the compile-time type of the variable. Here, 'animal' is declared as Animal, so Animal.sound() is called, printing "Animal sound".

❓ Predict Output

intermediate

Extension function call with subclass reference

What will this Kotlin program print?

Kotlin

open class Vehicle
class Car : Vehicle()

fun Vehicle.describe() = "Vehicle"
fun Car.describe() = "Car"

fun main() {
    val car = Car()
    println(car.describe())
}

A Vehicle

B Compilation error

C Runtime exception

D Car

💡 Hint

The variable type is Car, so Car's extension function is called.

Since 'car' is declared as Car, the extension function for Car is called, printing "Car".

🔧 Debug

advanced

Why does this extension function call print unexpected output?

Consider this Kotlin code. Why does it print "Base extension" instead of "Derived extension"?

Kotlin

open class Base
class Derived : Base()

fun Base.info() = "Base extension"
fun Derived.info() = "Derived extension"

fun printInfo(b: Base) {
    println(b.info())
}

fun main() {
    val d = Derived()
    printInfo(d)
}

A Extension functions are resolved statically by the declared type of the parameter 'b'.

B Derived.info() is private and cannot be called.

C The code has a runtime exception causing fallback to Base.info().

D Kotlin does not support extension functions on subclasses.

💡 Hint

Think about the type of 'b' inside printInfo function.

The parameter 'b' is declared as Base, so the extension function for Base is called, even if the actual object is Derived.

🧠 Conceptual

advanced

Understanding extension function dispatch in Kotlin

Which statement best describes how Kotlin resolves extension functions when called on an object?

A Extension functions are resolved at compile time using the declared type of the variable.

B Extension functions are resolved at runtime using the actual type of the object.

C Extension functions override member functions if they have the same signature.

D Extension functions can be dynamically dispatched like virtual methods.

💡 Hint

Consider if extension functions behave like normal class methods.

Kotlin resolves extension functions statically at compile time based on the declared type, not dynamically at runtime.

❓ Predict Output

expert

Output of extension function with nullable receiver and smart cast

What is the output of this Kotlin code?

Kotlin

fun String?.describe() = if (this == null) "Null string" else "String of length ${this.length}"

fun main() {
    val s: String? = null
    println(s.describe())

    val t: String? = "hello"
    println(t.describe())
}

A Null string\nNull string

B Null string\nString of length 5

C Compilation error due to nullable receiver

D Runtime exception due to null access

💡 Hint

Extension functions can have nullable receivers and handle null safely.

The extension function checks if 'this' is null and returns "Null string" or the length accordingly. So it prints "Null string" then "String of length 5".