Nullable receiver extensions in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an extension function on a nullable String that returns its length or 0 if null.

Kotlin

fun String?.nullableLength(): Int = this?.[1] ?: 0

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

Alength

Bsize

Ccount

Dlength()

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses like length() which is not a function

Using size which is not a property of String

Not using the safe call operator

The extension function uses the safe call operator ?. to access the length property if the string is not null. If it is null, it returns 0.

2fill in blank

medium

Complete the code to safely convert a nullable String to uppercase using an extension function.

Kotlin

fun String?.toUpperCaseSafe(): String = this?.[1]() ?: ""

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

AtoUpper

BtoUpperCase

Cuppercase

Duppercase()

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated toUpperCase() instead of uppercase()

Adding parentheses inside the blank like uppercase()

Using incorrect function names like toUpper

In modern Kotlin, strings are converted to uppercase using the uppercase() function. The blank is filled with uppercase since the parentheses follow the blank in the template.

3fill in blank

hard

Fix the error in the extension function that returns the first character of a nullable String or '-' if null.

Kotlin

fun String?.firstCharOrDash(): Char = this?.[1] ?: '-'

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

Afirst()

Bget(0)()

Cget(0)

Dfirst

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses (e.g., 'first') causing syntax error

Using malformed 'get(0)' or 'get(0)()'

Confusing with non-existent 'first' property

first() is a standard library function on CharSequence (including String) that returns the first Char. Since the template lacks parentheses after the blank, include them in the blank: first().

4fill in blank

hard

Fill both blanks to create an extension function that returns the last character of a nullable String or '?' if null or empty.

Kotlin

fun String?.lastCharOrQuestion(): Char = if (this != null && this.[1] > 0) this[[2]] else '?'

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

Alength

Blast()

Clength - 1

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using size instead of length for String

Using last() without proper null/empty handling

Forgetting to subtract 1 from length for the last index

The function checks if the string is not null and has length greater than zero. Then it returns the last character using Kotlin's indexing syntax [length - 1].

5fill in blank

hard

Fill all three blanks to create an extension function that returns a map of characters to their counts for a nullable String, or an empty map if null.

Kotlin

fun String?.charCountMap(): Map<Char, Int> = this?.[1]()?.[2] { it -> it }?.eachCount() ?: [3]()

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

AtoList

BgroupingBy

CemptyMap

DtoCharArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using toList() on String directly (works but not optimal; toCharArray() is fine)

Forgetting eachCount() after groupingBy

Returning null or wrong empty collection instead of emptyMap()

Convert the string to a CharArray with toCharArray(), group by character identity with groupingBy { it }, call eachCount() to get Map<Char, Int>, or return emptyMap() if null.