Extensions on built-in types in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extension function on String

What is the output of this Kotlin code that uses an extension function on a String?

Kotlin

fun String.addExclamation(): String {
    return this + "!"
}

fun main() {
    val greeting = "Hello"
    println(greeting.addExclamation())
}

A Compilation error

B Hello

C HELLO!

D Hello!

💡 Hint

Look at how the extension function adds an exclamation mark to the original string.

The extension function addExclamation appends an exclamation mark to the original string. So "Hello" becomes "Hello!".

❓ Predict Output

intermediate

Extension function on List with condition

What is the output of this Kotlin code that defines an extension function on List to sum only even numbers?

Kotlin

fun List<Int>.sumEven(): Int {
    return this.filter { it % 2 == 0 }.sum()
}

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    println(numbers.sumEven())
}

A 9

B 10

C 6

D Compilation error

💡 Hint

Sum only the even numbers: 2 and 4.

The extension function filters the list to keep only even numbers (2 and 4) and sums them: 2 + 4 = 6.

🔧 Debug

advanced

Why does this extension function cause a compilation error?

This Kotlin code tries to add an extension function to Int to check if it is prime. Why does it cause a compilation error?

Kotlin

fun Int.isPrime(): Boolean {
    if (this < 2) return false
    for (i in 2 until this) {
        if (this % i == 0) return false
    }
    return true
}

fun main() {
    val number = 7
    println(number.isPrime())
}

A No error, code compiles and prints true

B Error because extension functions cannot be defined on Int

C Error because 'until' is not imported

D Error because 'this' cannot be used in extension functions

💡 Hint

Extension functions can be defined on any type including Int.

Extension functions can be defined on Int. The code compiles and correctly prints true for 7 being prime.

📝 Syntax

advanced

Identify the syntax error in this extension function

Which option shows the correct syntax for an extension function on String that returns the first character?

Kotlin

fun String.firstChar(): Char {
    return this[0]
}

A fun String.firstChar: Char { return this[0] }

B fun String.firstChar() = this[0]

C fun firstChar(this: String): Char = this[0]

D fun String.firstChar() { return this[0] }

💡 Hint

Extension functions must have the receiver type before the function name.

Option B uses correct syntax with expression body. Option B is a normal function, not extension. Option B misses parentheses. Option B has a block but no return type declared.

🚀 Application

expert

Using extension functions to add behavior to built-in types

You want to add an extension function to List that returns a single string with all elements joined by a comma and space. Which option produces the correct output for listOf("apple", "banana", "cherry")?

Kotlin

fun List<String>.toCommaSeparated(): String {
    // implementation
}

fun main() {
    val fruits = listOf("apple", "banana", "cherry")
    println(fruits.toCommaSeparated())
}

A return this.joinToString(", ")

B return this.reduce { acc, s -> acc + ", " + s }

C return this.joinToString(separator = ",")

D return this.fold("") { acc, s -> acc + s + ", " }

💡 Hint

Use the built-in joinToString function with proper separator.

Option A uses joinToString with ", " separator correctly. Option A works for non-empty lists but throws an exception on empty lists. Option A uses "," without space. Option A adds extra comma at end.

Practice

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

easy

A. To create new classes that inherit from built-in types

B. To add new functions to existing types without modifying their source code

C. To override existing functions in built-in types

D. To convert one type to another automatically

Solution

1. Step 1: Understand extension functions concept

   Extension functions allow adding new functions to existing types without changing their original code.

2. Step 2: Compare options with concept

   Only To add new functions to existing types without modifying their source code correctly describes this purpose. Options B and C talk about inheritance and overriding, which are different concepts. To convert one type to another automatically is unrelated.

3. Final Answer:

   To add new functions to existing types without modifying their source code -> Option B

4. Quick Check:

   Extension functions add new behavior without changing original code [OK]

Hint: Remember: extensions add functions, not change classes [OK]

Common Mistakes:

• Confusing extension functions with inheritance
• Thinking extensions override existing methods
• Believing extensions convert types automatically

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

easy

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

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

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

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

Solution

1. Step 1: Recall extension function syntax

   Extension functions are declared as fun TypeName.functionName(). Here, Int.isEven() is correct.

2. Step 2: Check each option

   fun Int.isEven(): Boolean { return this % 2 == 0 } matches the correct syntax. fun isEven(): Boolean { return this % 2 == 0 } misses the receiver type. fun Int.isEven: Boolean = this % 2 == 0 misses parentheses. fun isEven(Int): Boolean = Int % 2 == 0 has wrong parameter syntax.

3. Final Answer:

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

4. Quick Check:

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

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

Common Mistakes:

• Omitting the receiver type before function name
• Missing parentheses after function name
• Using parameter instead of receiver for extension

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

fun String.addExclamation() = this + "!"

fun main() {
    val word = "Hello"
    println(word.addExclamation())
}

medium

A. Hello

B. Hello!!

C. Compilation error

D. Hello!

Solution

1. Step 1: Understand the extension function behavior

   The extension function addExclamation adds an exclamation mark to the string.

2. Step 2: Trace the main function output

   Calling word.addExclamation() returns "Hello!" which is printed.

3. Final Answer:

   Hello! -> Option D

4. Quick Check:

   Extension adds "!" so output is "Hello!" [OK]

Hint: Extension functions act like normal methods on the type [OK]

Common Mistakes:

• Expecting original string unchanged
• Thinking extension adds multiple exclamations
• Assuming syntax error due to extension

4. Identify the error in this Kotlin extension function and fix it:

fun Int.isPositive {
    return this > 0
}

medium

A. Missing parentheses after function name; should be fun Int.isPositive()

B. Extension functions cannot return Boolean

C. The receiver type Int is invalid for extensions

D. The return statement is not allowed in extension functions

Solution

1. Step 1: Check function declaration syntax

   Extension functions must have parentheses after the function name, even if no parameters.

2. Step 2: Identify the fix

   Add parentheses: fun Int.isPositive(). Other options are incorrect because Boolean return is allowed, Int is valid receiver, and return statements are allowed.

3. Final Answer:

   Missing parentheses after function name; should be fun Int.isPositive() -> Option A

4. Quick Check:

   Extension functions need () after name [OK]

Hint: Always add () after extension function name, even if no parameters [OK]

Common Mistakes:

• Omitting parentheses after function name
• Thinking extensions can't return values
• Confusing receiver type validity

5. You want to create an extension function wordCount for String that returns the number of words separated by spaces. Which implementation correctly handles multiple spaces and empty strings?

hard

A. fun String.wordCount() = this.split(" ").size

B. fun String.wordCount() = this.count { it == ' ' }

C. fun String.wordCount() = this.trim().split(" ").filter { it.isNotEmpty() }.size

D. fun String.wordCount() = this.length

Solution

1. Step 1: Understand the problem requirements

   We need to count words separated by spaces, ignoring extra spaces and empty strings.

2. Step 2: Analyze each option

   fun String.wordCount() = this.split(" ").size splits by space but counts empty strings if multiple spaces exist. fun String.wordCount() = this.trim().split(" ").filter { it.isNotEmpty() }.size trims, splits, filters out empty strings, then counts, correctly handling multiple spaces and empty strings. fun String.wordCount() = this.length returns string length, unrelated. fun String.wordCount() = this.count { it == ' ' } counts spaces, not words.

3. Final Answer:

   fun String.wordCount() = this.trim().split(" ").filter { it.isNotEmpty() }.size -> Option C

4. Quick Check:

   Trim, split, filter empty = correct word count [OK]

Hint: Trim and filter empty strings after split to count words correctly [OK]

Common Mistakes:

• Counting empty strings as words
• Using string length instead of word count
• Counting spaces instead of words