Extensions vs member functions priority in Kotlin - Hands-On Comparison

Extensions vs member functions priority

📖 Scenario: Imagine you have a class representing a simple device. You want to add a function to show its status. Kotlin allows you to add functions directly inside the class or add extension functions outside the class. But what happens if both have the same function name?

🎯 Goal: You will create a class with a member function and an extension function with the same name. Then you will call the function to see which one Kotlin uses by default.

📋 What You'll Learn

Create a class called Device with a member function status() that returns the string "Member function status".

Create an extension function status() for the Device class that returns the string "Extension function status".

Create an instance of Device called device.

Call device.status() and print the result.

💡 Why This Matters

🌍 Real World

In real apps, you might want to add new functions to existing classes without changing their code. Kotlin extensions let you do that, but member functions always take priority if they exist.

💼 Career

Understanding how Kotlin resolves functions helps you avoid bugs and write clearer code, which is important for Android development and other Kotlin-based projects.

1

Create the Device class with a member function

Create a class called Device with a member function status() that returns the string "Member function status".

Kotlin

# Create the Device class with a member function status
// Your code here

Hint

Define a class named Device. Inside it, write a function status that returns the exact string "Member function status".

2

Add an extension function status() for Device

Create an extension function called status() for the Device class that returns the string "Extension function status".

Kotlin

class Device {
    fun status(): String {
        return "Member function status"
    }
}

// Add an extension function status for Device
// Your code here

Hint

Write a function outside the class with the syntax fun Device.status(): String. It should return "Extension function status".

3

Create an instance of Device called device

Create a variable called device and assign it a new instance of the Device class.

Kotlin

class Device {
    fun status(): String {
        return "Member function status"
    }
}

fun Device.status(): String {
    return "Extension function status"
}

// Create an instance of Device called device
// Your code here

Hint

Use val device = Device() to create the instance.

4

Call device.status() and print the result

Call the status() function on the device instance and print the returned string using println.

Kotlin

class Device {
    fun status(): String {
        return "Member function status"
    }
}

fun Device.status(): String {
    return "Extension function status"
}

val device = Device()

// Print the result of device.status()
// Your code here

Hint

Use println(device.status()) to print the output. Notice which function is called.

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