Extensions vs member functions priority in Kotlin - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of member function vs extension function

What is the output of this Kotlin code snippet?

Kotlin

class Sample {
    fun greet() = "Member"
}

fun Sample.greet() = "Extension"

fun main() {
    val s = Sample()
    println(s.greet())
}

A Extension

B Member

C Compilation error

D Runtime exception

💡 Hint

Member functions have higher priority than extension functions when called on an instance.

In Kotlin, if a member function and an extension function have the same signature, the member function is called, not the extension.

❓ Predict Output

intermediate

Extension function called on nullable receiver

What is the output of this Kotlin code?

Kotlin

class Box {
    fun info() = "Member Box"
}

fun Box?.info() = "Extension Box?"

fun main() {
    val b: Box? = null
    println(b.info())
}

A Extension Box?

B Member Box

C NullPointerException

D Compilation error

💡 Hint

Extension functions can be called on nullable receivers, but member functions cannot.

Since b is null, member function info() cannot be called. The extension function with nullable receiver is called instead.

🔧 Debug

advanced

Why does this extension function not override the member function?

Consider this Kotlin code. Why does calling foo() print "Member" instead of "Extension"?

Kotlin

open class Base {
    fun foo() = "Member"
}

fun Base.foo() = "Extension"

fun main() {
    val b: Base = Base()
    println(b.foo())
}

A Extension functions cannot override member functions; member functions always have priority.

B Extension functions override member functions only if marked with override keyword.

C Extension functions override member functions only if the class is open.

D Extension functions override member functions only if they have different signatures.

💡 Hint

Extension functions are resolved statically and do not override member functions.

Extension functions are resolved at compile time and do not override member functions. Member functions always have priority.

🧠 Conceptual

advanced

Extension function dispatch behavior

Which statement best describes how Kotlin resolves extension functions compared to member functions?

A Member functions are resolved statically, extension functions are resolved dynamically.

B Both extension and member functions are resolved dynamically at runtime based on the actual object type.

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

D Extension functions are resolved statically based on the declared type, member functions are resolved dynamically based on the runtime type.

💡 Hint

Think about how Kotlin decides which function to call when both member and extension functions exist.

Member functions are virtual and resolved at runtime based on the actual object type. Extension functions are resolved at compile time based on the declared type.

❓ Predict Output

expert

Output with extension function and inheritance

What is the output of this Kotlin program?

Kotlin

open class Parent {
    fun greet() = "Parent member"
}

class Child : Parent()

fun Parent.greet() = "Parent extension"
fun Child.greet() = "Child extension"

fun main() {
    val c: Parent = Child()
    println(c.greet())
}

A Child extension

B Parent extension

C Parent member

D Compilation error

💡 Hint

Member functions have priority over extension functions even when the runtime type is a subclass.

The variable c is declared as Parent, so extension function for Parent is considered. But member function greet() exists and has priority, so "Parent member" is printed.