Platform types and null safety in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a nullable String variable.

Kotlin

var name: String[1] = null

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

A?

B!

C(no suffix)

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!' or '!!' which are not used for declaring nullable types.

Leaving the type without '?' which means it cannot hold null.

In Kotlin, adding ? after a type makes it nullable, allowing it to hold null.

2fill in blank

medium

Complete the code to safely access the length of a nullable String.

Kotlin

val length = name[1].length ?: 0

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

A!!

B?

C:

D?.

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' which throws an exception if the value is null.

Using '?' alone which is not a valid operator.

Using ':' which is not an operator for property access.

The safe call operator ?. accesses a property only if the object is not null, otherwise returns null.

3fill in blank

hard

Fix the error in the code by choosing the correct operator to assert non-null.

Kotlin

val length = name[1].length

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

A?

B?.

C!!

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using '?' or '?.' which allow null but do not assert non-null.

Using ':' which is not an operator here.

The non-null assertion operator !! tells Kotlin the value is not null and throws an exception if it is.

4fill in blank

hard

Fill both blanks to create a platform type variable and safely access its length.

Kotlin

val platformName: String[1] = getJavaString()
val length = platformName[2].length ?: 0

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

A?

B!!

C?.

D(no suffix)

Attempts:

3 left

💡 Hint

Common Mistakes

Adding '?' to platform type which is not needed.

Using '!!' which can cause exceptions if null.

Platform types come from Java and Kotlin treats them as non-null or nullable. We leave the type without '?' to indicate platform type. Use safe call ?. to access length safely.

5fill in blank

hard

Fill all three blanks to filter a list of platform type Strings for non-null values and get their lengths.

Kotlin

val javaList: List<String[1]> = getJavaList()
val lengths = javaList.filterNotNull().map { it[2].length }.filter { it [3] 0 }

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

A?

B!!

C>

D(no suffix)

Attempts:

3 left

💡 Hint

Common Mistakes

Adding '?' to platform type list.

Not asserting non-null before accessing length.

Using wrong comparison operator in filter.

Platform types have no explicit nullability mark, so blank 1 is empty. Use !! to assert non-null inside map. Then filter lengths greater than zero.