Extension functions in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the correct extension function syntax

Which option correctly defines an extension function isEven for Int that returns true if the number is even?

A extension fun Int.isEven(): Boolean { return this % 2 == 0 }

B fun Int.isEven: Boolean = this % 2 == 0

C fun isEven(this Int): Boolean = this % 2 == 0

D fun Int.isEven(): Boolean = this % 2 == 0

💡 Hint

Remember, extension functions use the syntax: fun ReceiverType.functionName() { ... }

Option D correctly uses the Kotlin extension function syntax with the receiver type before the function name and a proper function body returning a Boolean.

❓ ui_behavior

intermediate

Extension function effect on UI component

Given this extension function for TextView in Android Kotlin:

fun TextView.makeRed() { this.setTextColor(android.graphics.Color.RED) }

What will happen when you call myTextView.makeRed() in an activity?

A The text color of myTextView changes to red

B The background color of myTextView changes to red

C The app crashes with a NullPointerException

D Nothing happens because extension functions cannot modify UI

💡 Hint

Extension functions can call existing methods on the receiver object.

The extension function calls setTextColor on the TextView instance, changing its text color to red.

❓ lifecycle

advanced

Extension function and lifecycle impact

Consider an extension function on Activity that shows a toast message:

fun Activity.showWelcome() { Toast.makeText(this, "Welcome!", Toast.LENGTH_SHORT).show() }

What is a potential lifecycle-related issue when calling showWelcome() inside onCreate() of an activity?

A The toast may not show if the activity is not fully created yet

B The toast will cause a memory leak if called in onCreate

C The extension function will crash because Toast requires a Fragment

D There is no issue; the toast always shows correctly

💡 Hint

Consider when UI elements are ready during the activity lifecycle.

If called too early, the UI might not be ready to display the toast properly, so the toast might not appear as expected.

❓ navigation

advanced

Extension function for navigation

You have this extension function for Fragment to navigate to another fragment:

fun Fragment.goToDetails() { findNavController().navigate(R.id.detailsFragment) }

What happens if you call goToDetails() in a fragment that is not attached to a NavController?

A Navigates to detailsFragment successfully

B Causes a compile-time error

C Throws an IllegalStateException at runtime

D Does nothing silently

💡 Hint

findNavController() requires the fragment to be attached to a NavHost.

If the fragment is not attached to a NavController, findNavController() throws an IllegalStateException at runtime.

🔧 Debug

expert

Debugging extension function unexpected behavior

Given this extension function:

fun String.removeFirstChar(): String = this.substring(1)

What happens if you call "".removeFirstChar() (empty string)?

A Returns an empty string

B Throws StringIndexOutOfBoundsException

C Returns null

D Returns the original empty string

💡 Hint

Check what substring(1) does on an empty string.

Calling substring(1) on an empty string causes StringIndexOutOfBoundsException because index 1 is out of range.

Practice

1. What is the main purpose of an extension function in Kotlin?

easy

A. To override existing functions in a class

B. To create a new subclass from an existing class

C. To add new functions to existing classes without modifying them

D. To delete functions from a class

Solution

1. Step 1: Understand extension functions

   Extension functions let you add new functions to classes without changing their original code.

2. Step 2: Compare options

   Only To add new functions to existing classes without modifying them correctly describes this behavior; others describe subclassing, overriding, or deleting, which are different concepts.

3. Final Answer:

   To add new functions to existing classes without modifying them -> Option C

4. Quick Check:

   Extension function = Add new functions [OK]

Hint: Extension functions add features without changing original class [OK]

Common Mistakes:

• Confusing extension functions with inheritance
• Thinking extension functions override existing methods
• Believing extension functions can remove class methods

2. Which of the following is the correct syntax to declare an extension function isEven() for Int in Kotlin?

easy

A. fun isEven(): Boolean { return this % 2 == 0 }

B. fun Int.isEven(): Boolean { return this % 2 == 0 }

C. fun Int.isEven: Boolean { return this % 2 == 0 }

D. fun Int.isEven() Boolean = this % 2 == 0

Solution

1. Step 1: Check extension function syntax

   Extension functions are declared with fun ClassName.functionName(), so fun Int.isEven() is correct.

2. Step 2: Validate return type and body

   fun Int.isEven(): Boolean { return this % 2 == 0 } correctly uses parentheses and return type with a proper function body.

3. Final Answer:

   fun Int.isEven(): Boolean { return this % 2 == 0 } -> Option B

4. Quick Check:

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

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

Common Mistakes:

• Omitting the receiver type before function name
• Missing parentheses after function name
• Incorrect return type declaration

3. What will be the output of this Kotlin code?

fun String.addHello() = "Hello, $this!"

fun main() {
  val name = "Anna"
  println(name.addHello())
}

medium

A. Hello, Anna!

B. AnnaHello, !

C. Hello, !

D. Compilation error

Solution

1. Step 1: Understand the extension function

   The function addHello() adds "Hello, " before the string and an exclamation mark after.

2. Step 2: Apply function to variable

   Calling name.addHello() where name is "Anna" returns "Hello, Anna!".

3. Final Answer:

   Hello, Anna! -> Option A

4. Quick Check:

   Extension adds prefix and suffix correctly [OK]

Hint: Extension functions run like normal methods on instances [OK]

Common Mistakes:

• Misreading string interpolation syntax
• Assuming extension modifies original string
• Expecting output without exclamation mark

4. Identify the error in this Kotlin extension function code:

fun Int.isPositive() {
  return this > 0
}

medium

A. Function must declare a return type Boolean

B. The function body should not use return

C. The keyword 'this' cannot be used in extension functions

D. Extension function cannot be declared on Int

Solution

1. Step 1: Check function return type

   The function returns a Boolean expression but does not declare a return type, which is required.

2. Step 2: Confirm correct syntax

   Adding : Boolean after the function name fixes the error.

3. Final Answer:

   Function must declare a return type Boolean -> Option A

4. Quick Check:

   Return type needed for non-Unit functions [OK]

Hint: Declare return type when function returns a value [OK]

Common Mistakes:

• Omitting return type for Boolean-returning functions
• Thinking 'this' is disallowed in extensions
• Confusing Unit functions with value-returning ones

5. You want to add an extension function toTitleCase() to String that capitalizes the first letter of each word. Which implementation correctly achieves this?

hard

A. fun String.toTitleCase() = this.toUpperCase()

B. fun String.toTitleCase() = this.lowercase()

C. fun String.toTitleCase() = this.replaceFirstChar { it.uppercase() }

D. fun String.toTitleCase() = split(" ").joinToString(" ") { it.capitalize() }

Solution

1. Step 1: Understand title case requirement

   Title case means capitalizing the first letter of each word, not the whole string or just the first letter.

2. Step 2: Analyze options

   fun String.toTitleCase() = split(" ").joinToString(" ") { it.capitalize() } splits the string by spaces, capitalizes each word, then joins them back, correctly implementing title case.

3. Final Answer:

   fun String.toTitleCase() = split(" ").joinToString(" ") { it.capitalize() } -> Option D

4. Quick Check:

   Title case = capitalize each word [OK]

Hint: Split words, capitalize each, then join for title case [OK]

Common Mistakes:

• Using uppercase() on whole string instead of each word
• Capitalizing only first character of entire string
• Ignoring spaces between words