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.

Practice

1. In Kotlin, when a class has a member function and an extension function with the same name, which one is called by default?

easy

A. The member function of the class

B. The extension function

C. Both are called

D. It causes a compile-time error

Solution

1. Step 1: Understand member vs extension functions

   Member functions are defined inside the class, while extension functions are defined outside to add functionality.

2. Step 2: Check Kotlin's call priority

   Kotlin always prefers member functions over extension functions when both have the same name.

3. Final Answer:

   The member function of the class -> Option A

4. Quick Check:

   Member functions have priority = A [OK]

Hint: Member functions override extensions with same name [OK]

Common Mistakes:

• Thinking extension functions override member functions
• Assuming both functions run together
• Believing it causes a compile error

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 are declared as fun ClassName.functionName().

2. Step 2: Match syntax with options

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

3. Final Answer:

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

4. Quick Check:

   Extension syntax = D [OK]

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

Common Mistakes:

• Placing function name before class name
• Using 'extension' keyword incorrectly
• Wrong function declaration order

3. Consider the following Kotlin code:

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

fun Box.size() = "Extension size"

fun main() {
    val box = Box()
    println(box.size())
}

What is the output when this code runs?

medium

A. Compilation error

B. Member size

C. Extension size

D. Runtime exception

Solution

1. Step 1: Identify member and extension functions

   Class Box has a member function size() returning "Member size". An extension function with the same name returns "Extension size".

2. Step 2: Determine which function is called

   Kotlin calls the member function when both exist, so box.size() calls the member function.

3. Final Answer:

   Member size -> Option B

4. Quick Check:

   Member function called over extension = C [OK]

Hint: Member function called if name clashes [OK]

Common Mistakes:

• Expecting extension function output
• Thinking both outputs print
• Assuming compile error due to name clash

4. Given this Kotlin code:

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

fun Gadget.info() = "Extension info"

fun main() {
    val g: Gadget? = null
    println(g?.info())
}

What is the output when running this code?

medium

A. null

B. Member info

C. Extension info

D. NullPointerException

Solution

1. Step 1: Analyze nullable variable and safe call

   Variable g is null. The safe call operator ?. prevents calling functions on null.

2. Step 2: Understand safe call with extension/member functions

   Since g is null, g?.info() returns null without calling any function.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Safe call on null returns null = B [OK]

Hint: Safe call on null returns null, no function called [OK]

Common Mistakes:

• Expecting function output despite null
• Thinking NullPointerException occurs
• Confusing extension and member call on null

5. You want to add a describe() function to class Device without modifying its source code. The class already has a member function describe(). How can you call the extension describe() function explicitly in Kotlin?

hard

A. You cannot call the extension if member exists with same name

B. Use (device as Device).describe() to call extension

C. Call extension with a wrapper function or different name

D. Use Device::describe.invoke(device) to call extension

Solution

1. Step 1: Understand member vs extension call priority

   Kotlin always calls member functions over extensions if names clash; extension cannot override member.

2. Step 2: Find workaround to call extension

   To call extension explicitly, rename it or call via a wrapper function with a different name.

3. Final Answer:

   Call extension with a wrapper function or different name -> Option C

4. Quick Check:

   Rename or wrap extension to call it = A [OK]

Hint: Rename extension or wrap it to call when member exists [OK]

Common Mistakes:

• Trying to cast to call extension directly
• Expecting extension to override member
• Using reflection incorrectly to call extension