Extension function syntax in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an extension function named greet for the String class that returns a greeting message.

Kotlin

fun String.[1](): String {
    return "Hello, $this!"
}

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

Agreet

Bhello

CsayHello

Dwelcome

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the function name after the receiver type.

Using a function name that is not descriptive.

The extension function name is greet, so the correct syntax is fun String.greet().

2fill in blank

medium

Complete the code to call the extension function greet on the string "World".

Kotlin

val message = "World".[1]()
println(message)

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

AsayHello

Bhello

Cwelcome

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a function name that was not defined as an extension.

Forgetting the parentheses when calling the function.

To call the extension function defined as greet, use "World".greet().

3fill in blank

hard

Fix the error in the extension function definition by completing the blank with the correct receiver type.

Kotlin

fun [1].greet(): String {
    return "Hi, $this!"
}

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

AInt

BDouble

CString

DBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong receiver type like Int or Double.

Omitting the receiver type entirely.

The extension function is meant for String, so the receiver type must be String.

4fill in blank

hard

Fill both blanks to create an extension function repeatPrint for Int that prints a message multiple times.

Kotlin

fun Int.[1](message: String) {
    for (i in 1..[2]) {
        println(message)
    }
}

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

ArepeatPrint

BtimesPrint

Cthis

DprintTimes

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong function name.

Using a fixed number instead of the receiver value in the loop.

The function name is repeatPrint (A). The number of repetitions is the receiver Int, so use this (C) in the loop.

5fill in blank

hard

Fill in the blanks to create an extension function isLongerThan for String that checks if the string length is greater than a given number.

Kotlin

fun String.[1](length: Int): Boolean {
    return this.[2] > length
}

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

AisLongerThan

Blength

Ccount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using count or size which are not properties of String.

Forgetting to compare the length with the parameter.

The function name is isLongerThan (A). To get the string length, use the length property (B).

Practice

1. What is the correct way to declare an extension function greet for the String type in Kotlin?

easy

A. extension fun String greet() { println("Hello") }

B. fun greet(String) { println("Hello") }

C. fun String.greet() { println("Hello, $this") }

D. fun greet() String { println("Hello") }

Solution

1. Step 1: Understand extension function syntax

   Extension functions use the syntax fun ReceiverType.functionName(). Here, String is the receiver type.

2. Step 2: Match the syntax with options

   fun String.greet() { println("Hello, $this") } correctly uses fun String.greet(). Other options do not follow this syntax.

3. Final Answer:

   fun String.greet() { println("Hello, $this") } -> Option C

4. Quick Check:

   Extension function syntax = fun ReceiverType.functionName() [OK]

Hint: Look for 'fun ReceiverType.functionName()' pattern [OK]

Common Mistakes:

• Using 'extension' keyword which doesn't exist
• Placing receiver type as function parameter
• Missing receiver type before function name

2. Which of the following is the correct syntax to declare an extension function square for Int that returns the square of the number?

easy

A. fun Int.square(): Int = this * this

B. fun Int.square() { this * this }

C. fun Int.square() = Int * Int

D. fun square(Int): Int = this * this

Solution

1. Step 1: Check extension function declaration with return type

   The syntax is fun ReceiverType.functionName(): ReturnType = expression. fun Int.square(): Int = this * this matches this.

2. Step 2: Verify usage of 'this' and return

   fun Int.square(): Int = this * this correctly uses this * this and returns Int. fun Int.square() { this * this } uses braces (block body) but misses return statement, thus returns Unit. fun Int.square() = Int * Int uses invalid Int * Int.

3. Final Answer:

   fun Int.square(): Int = this * this -> Option A

4. Quick Check:

   Extension function with return type = fun ReceiverType.functionName(): Type [OK]

Hint: Remember extension functions use receiver before dot and return type after parentheses [OK]

Common Mistakes:

• Placing receiver type as function parameter
• Using braces without return in expression body
• Incorrect return type placement

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

fun String.shout() = this.uppercase() + "!"

fun main() {
    val word = "hello"
    println(word.shout())
}

medium

A. hello!

B. HELLO!

C. Hello!

D. Compilation error

Solution

1. Step 1: Understand the extension function behavior

   The extension function shout converts the string to uppercase and adds an exclamation mark.

2. Step 2: Apply the function to the string "hello"

   Calling word.shout() returns "HELLO!" which is printed.

3. Final Answer:

   HELLO! -> Option B

4. Quick Check:

   Extension function output = uppercase + "!" [OK]

Hint: Check what extension function returns and apply to variable [OK]

Common Mistakes:

• Assuming original string changes
• Ignoring uppercase conversion
• Thinking extension functions modify original object

4. Identify the error in the following Kotlin extension function declaration:

fun Int.double {
    return this * 2
}

medium

A. Missing parentheses after function name

B. Cannot use 'this' in extension functions

C. Return statement not allowed in extension functions

D. Extension functions cannot be declared for Int

Solution

1. Step 1: Check function declaration syntax

   Functions must have parentheses after the name, even if no parameters are present. Here, double misses parentheses.

2. Step 2: Verify other parts

   Using this is valid in extension functions. Return statements are allowed. Extension functions can be declared for Int.

3. Final Answer:

   Missing parentheses after function name -> Option A

4. Quick Check:

   Function declaration requires parentheses () [OK]

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

Common Mistakes:

• Omitting parentheses in function declaration
• Misunderstanding 'this' usage
• Thinking return is disallowed

5. You want to add an extension function isEven() to Int? (nullable Int) that returns true if the number is even, and false if it's odd or null. Which is the correct way to declare this extension function?

hard

A. fun Int.isEven(): Boolean = this % 2 == 0 || this == null

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

C. fun Int?.isEven(): Boolean { if (this == null) return false else return this % 2 != 0 }

D. fun Int?.isEven(): Boolean = this != null && this % 2 == 0

Solution

1. Step 1: Understand nullable receiver type

   To extend nullable Int, use Int? as receiver type. fun Int?.isEven(): Boolean = this != null && this % 2 == 0 does this correctly.

2. Step 2: Check logic for null and even check

   fun Int?.isEven(): Boolean = this != null && this % 2 == 0 uses concise expression: returns false if null, true if even. fun Int?.isEven(): Boolean { if (this == null) return false else return this % 2 != 0 } is verbose but has incorrect logic (returns true for odd). fun Int.isEven(): Boolean = this % 2 == 0 || this == null incorrectly uses non-nullable receiver and wrong logic. fun Int.isEven(): Boolean = this % 2 == 0 ignores null.

3. Final Answer:

   fun Int?.isEven(): Boolean = this != null && this % 2 == 0 -> Option D

4. Quick Check:

   Nullable receiver uses '?' and checks null explicitly [OK]

Hint: Use '?' after type for nullable receiver and check null inside [OK]

Common Mistakes:

• Not using nullable receiver type for Int?
• Ignoring null checks inside function
• Using non-nullable receiver for nullable types