Extensions on built-in types in 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.

Kotlin

fun Int.isEven(): Boolean {
    return this [1] 2 == 0
}

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

A*

B/

C%

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using division (/) instead of modulus (%) causes wrong results.

The modulus operator (%) returns the remainder. If the remainder when divided by 2 is 0, the number is even.

2fill in blank

medium

Complete the code to define an extension function lastChar for String that returns its last character.

Kotlin

fun String.lastChar(): Char {
    return this[1]length - 1]
}

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

A<

B(

C{

D[

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses () instead of square brackets [] for indexing.

Square brackets [] are used to access characters by index in a String.

3fill in blank

hard

Fix the error in the extension function that tries to double each element in a list of integers.

Kotlin

fun List<Int>.doubleElements(): List<Int> {
    return this.map { it [1] 2 }
}

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of * results in adding 2 instead of doubling.

To double each element, multiply it by 2 using the * operator.

4fill in blank

hard

Fill both blanks to create an extension function filterShortWords for List<String> that returns words shorter than 5 characters.

Kotlin

fun List<String>.filterShortWords(): List<String> {
    return this.filter { word -> word.[1] < [2] }
}

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

Alength

B5

Ccount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using count or size instead of length for strings.

Use the length property to get the number of characters in a word and compare it to 5.

5fill in blank

hard

Fill all three blanks to create an extension function toMapByFirstChar for List<String> that returns a map grouping words by their first uppercase character.

Kotlin

fun List<String>.toMapByFirstChar(): Map<Char, List<String>> {
    return this.groupBy { it[1]().[2](0) }
        .mapKeys { it.key.[3]() }
}

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

AtoUpperCase

Buppercase

Cfirst

DuppercaseChar

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated 'toUpperCase()' instead of 'uppercase()'.

Confusing string and char uppercase functions.

Use 'uppercase()' to convert the string to uppercase, 'first' to get the first character, and 'uppercaseChar()' to convert a character to uppercase.

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