Extension functions in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an extension function isEven for Int that returns true if the number is even.

Android Kotlin

fun Int.[1](): Boolean = this % 2 == 0

Drag options to blanks, or click blank then click option'

AisEven

BevenCheck

CcheckEven

DisOdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that does not describe the purpose clearly.

Forgetting to declare the function as an extension on Int.

The extension function name should be isEven to clearly indicate it checks if the number is even.

2fill in blank

medium

Complete the code to call the extension function isEven on the integer number.

Android Kotlin

val number = 10
val result = number.[1]()

Drag options to blanks, or click blank then click option'

AcheckEven

BisEven

CisOdd

DevenCheck

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call the function without parentheses.

Using a wrong function name.

You call the extension function by using the dot notation on the variable: number.isEven().

3fill in blank

hard

Fix the error in the extension function that tries to add a greet function to String but is missing the receiver type.

Android Kotlin

fun [1].greet() = "Hello, $this!"

Drag options to blanks, or click blank then click option'

AString

Bthis

Cfun

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the receiver type causes a syntax error.

Placing the receiver type after the function name.

Extension functions must specify the receiver type before the function name, like fun String.greet().

4fill in blank

hard

Fill both blanks to create an extension function repeatText for String that repeats the string n times.

Android Kotlin

fun String.[1](n: Int): String = this.[2](n)

Drag options to blanks, or click blank then click option'

ArepeatText

B*

C+

Drepeat

Attempts:

3 left

💡 Hint

Common Mistakes

Using operators like * or + directly on strings without the repeat function.

Choosing a function name that does not match the task.

The function name is repeatText. To repeat a string in Kotlin, use the repeat(n) function, but since this is an extension, we call this.repeat(n).

5fill in blank

hard

Fill all three blanks to create an extension function toTitleCase for String that capitalizes the first letter and makes the rest lowercase.

Android Kotlin

fun String.[1](): String = this.substring(0,1).[2]() + this.substring(1).[3]()

Drag options to blanks, or click blank then click option'

AtoTitleCase

Buppercase

Ccapitalize

Dlowercase

Attempts:

3 left

💡 Hint

Common Mistakes

Using capitalize() which capitalizes the first letter but leaves the rest unchanged.

Not converting the rest of the string to lowercase.

The function name is toTitleCase. Use substring(0,1).uppercase() to capitalize the first character and lowercase() on the rest with substring(1).

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