Extensions resolved statically in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the extension function on a String.

Kotlin

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

fun main() {
    val name = "Alice"
    println(name.greet())
}

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Aname

Bthis

Cit

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name like 'name' instead of 'this' inside the extension function.

Using 'it' which is for lambda implicit parameter, not extension receiver.

In Kotlin extension functions, 'this' refers to the receiver object, here the String instance.

2fill in blank

medium

Complete the code to call the correct extension function based on the variable type.

Kotlin

fun String.describe() = "String extension"
fun Int.describe() = "Int extension"

fun main() {
    val x: Any = 42
    println((x as [1]).describe())
}

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AInt

BString

CAny

DNumber

Attempts:

3 left

💡 Hint

Common Mistakes

Casting to 'Any' or 'Number' which do not have the 'describe' extension.

Casting to 'String' which causes a runtime error.

Casting 'x' to Int allows calling the Int extension function 'describe()'.

3fill in blank

hard

Fix the error in the extension function call to use the correct receiver type.

Kotlin

open class Base
class Derived : Base()

fun Base.foo() = "Base"
fun Derived.foo() = "Derived"

fun printFoo(b: Base) {
    println(b.[1]())
}

fun main() {
    val d = Derived()
    printFoo(d)
}

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Abaz

Bbar

Cfoo

Dqux

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name not defined as an extension causes a compile error.

Expecting Derived.foo() to be called dynamically, but Kotlin resolves extensions statically.

Extension functions are resolved statically by the declared type, so 'foo' is called on Base type.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths, filtering words longer than 3 characters.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val lengths = {word: [1] for word in words if word [2] 3}

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Alength

Bcount

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'count' which is a function, not a property.

Using '<' which filters shorter words instead of longer.

Use 'length' property to get word length and '>' to filter words longer than 3.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, filtering words with length greater than 3.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val result = mapOf([1] to [2] for word in words if word.[3] > 3)

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Aword.uppercase()

Bword.length

Clength

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'word' instead of 'word.uppercase()' for keys.

Using 'word' instead of 'word.length' for values.

Using 'word.length' without '.length' property in the condition.

Use 'word.uppercase()' as key, 'word.length' as value, and 'length' property to filter words longer than 3.