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".

Practice

1. What does it mean that Kotlin extension functions are resolved statically?

easy

A. Extension functions override member functions dynamically.

B. The extension function called depends on the variable's declared type, not the actual object type.

C. The extension function is chosen based on the runtime type of the object.

D. Extension functions can change the original class's source code.

Solution

1. Step 1: Understand static resolution

   Static resolution means decisions are made at compile time, not runtime.

2. Step 2: Apply to extension functions

   Kotlin chooses the extension function based on the variable's declared type, ignoring the actual object type.

3. Final Answer:

   The extension function called depends on the variable's declared type, not the actual object type. -> Option B

4. Quick Check:

   Extensions resolved statically = based on declared type [OK]

Hint: Remember: extensions use variable type, not object type [OK]

Common Mistakes:

• Thinking extension functions override member functions dynamically
• Assuming extension functions change the original class
• Confusing static with dynamic dispatch

2. Which of the following is the correct syntax to declare an extension function greet for class Person in Kotlin?

easy

A. fun greet() Person { println("Hello") }

B. fun greet(Person) { println("Hello") }

C. extension fun Person.greet() { println("Hello") }

D. fun Person.greet() { println("Hello") }

Solution

1. Step 1: Recall extension function syntax

   Extension functions use the syntax: fun ClassName.functionName() { ... }

2. Step 2: Match syntax to options

   fun Person.greet() { println("Hello") } matches the correct syntax: fun Person.greet() { println("Hello") }

3. Final Answer:

   fun Person.greet() { println("Hello") } -> Option D

4. Quick Check:

   Extension syntax = fun ClassName.functionName() [OK]

Hint: Extension syntax: fun ClassName.functionName() [OK]

Common Mistakes:

• Using 'extension' keyword which doesn't exist
• Incorrect parameter placement
• Missing receiver type before function name

3. Consider the code:

open class Base
class Derived : Base()

fun Base.foo() = "Base"
fun Derived.foo() = "Derived"

fun printFoo(b: Base) {
    println(b.foo())
}

fun main() {
    val d: Derived = Derived()
    printFoo(d)
}

What is the output when main() runs?

medium

A. Compilation error

B. Derived

C. Base

D. Runtime exception

Solution

1. Step 1: Analyze parameter type in printFoo

   printFoo takes a parameter of type Base, so inside printFoo, b is of type Base.

2. Step 2: Understand extension resolution

   Extension functions are resolved statically by the declared type, so b.foo() calls Base.foo(), not Derived.foo().

3. Final Answer:

   Base -> Option C

4. Quick Check:

   Extension call depends on declared type = Base [OK]

Hint: Extension call uses parameter type, not actual object type [OK]

Common Mistakes:

• Expecting Derived.foo() to be called dynamically
• Confusing extension with member function dispatch
• Assuming runtime type affects extension resolution

4. Given the code below, what is the error and how to fix it?

open class Animal
class Dog : Animal()

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

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

medium

A. No error; output is "Some sound"

B. Compilation error: extension functions cannot be overloaded

C. Runtime error: unresolved extension function

D. Output is "Bark" but should be "Some sound"

Solution

1. Step 1: Check for compilation errors

   Extension functions can be overloaded by receiver type; no syntax error here.

2. Step 2: Understand output based on static resolution

   Variable animal is declared as Animal, so animal.sound() calls Animal.sound(), outputting "Some sound".

3. Final Answer:

   No error; output is "Some sound" -> Option A

4. Quick Check:

   Extension resolved by declared type = Animal.sound() [OK]

Hint: Extensions overload by receiver type; call depends on variable type [OK]

Common Mistakes:

• Expecting Dog.sound() to be called dynamically
• Thinking extension functions cause runtime errors
• Confusing extension overloading with overriding

5. You want to add an extension function describe() to a class hierarchy where Cat inherits from Animal. You want describe() to print "Animal description" when called on an Animal variable, and "Cat description" when called on a Cat variable. Given Kotlin's static resolution of extensions, how should you design your code to achieve this behavior?

hard

A. Declare two extension functions: one for Animal and one for Cat; call describe() on variables typed accordingly.

B. Override describe() as a member function in Cat and Animal classes.

C. Use a when expression inside a single extension function on Animal to check the runtime type.

D. Use reflection to call the correct extension function at runtime.

Solution

1. Step 1: Understand static resolution impact

   Extension functions are chosen by variable type, so different extensions for Animal and Cat can coexist.

2. Step 2: Design with two extensions and variable types

   Define describe() extensions for Animal and Cat separately, then call describe() on variables declared as Animal or Cat to get different outputs.

3. Final Answer:

   Declare two extension functions: one for Animal and one for Cat; call describe() on variables typed accordingly. -> Option A

4. Quick Check:

   Different extensions + variable types = desired output [OK]

Hint: Use separate extensions and variable types to get different outputs [OK]

Common Mistakes:

• Trying to override extensions as member functions
• Relying on runtime type for extension dispatch
• Using reflection unnecessarily