Nullable types with ? suffix 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 an exclamation mark instead of a question mark.

Leaving the type non-nullable and assigning 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

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 null.

Using just a dot which causes a compile error on nullable types.

The safe call operator ?. allows accessing a property or method only if the object is not null.

3fill in blank

hard

Fix the error in the code to assign a nullable String to a non-nullable variable.

Kotlin

val nonNullName: String = name[1]

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

A!!

B?.toString()

C?

D?: ""

Attempts:

3 left

💡 Hint

Common Mistakes

Using safe call ?. which returns nullable type.

Using ?: "" which is for providing default values.

The !! operator asserts that the value is not null and throws an exception if it is null.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths only if length is greater than 3.

Kotlin

val lengths = mapOf([1] to [2]).filter { it.value > 3 }

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

A"hello"

B"hello".length

C"hi"

D"hi".length

Attempts:

3 left

💡 Hint

Common Mistakes

Using a word shorter than 4 characters which will be filtered out.

Using the length property incorrectly.

The map pairs a word with its length. We use the word "hello" and its length.

5fill in blank

hard

Fill all three blanks to create a nullable variable, check if it is null, and provide a default value.

Kotlin

val input: String[1] = null
val result = input [2] "default"
println(result[3])

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

A?

B?:

C?.length

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Not marking the variable as nullable.

Using !! which throws exception if null.

Not using safe call operator before accessing length.

The variable is nullable with ?. The Elvis operator ?: provides a default if null. The safe call ?. accesses length safely.

Practice

1. What does adding a ? after a type in Kotlin mean?

easy

A. The variable is immutable.

B. The variable can hold a null value.

C. The variable is a constant.

D. The variable is a function.

Solution

1. Step 1: Understand Kotlin type suffix

   In Kotlin, adding ? after a type means the variable can hold null.

2. Step 2: Compare with non-nullable types

   Without ?, variables cannot be null and will cause errors if assigned null.

3. Final Answer:

   The variable can hold a null value. -> Option B

4. Quick Check:

   Nullable type = variable can be null [OK]

Hint: Remember: ? means nullable type in Kotlin [OK]

Common Mistakes:

• Thinking ? makes variable immutable
• Confusing ? with constant declaration
• Assuming ? means function type

2. Which of the following is the correct syntax to declare a nullable String variable in Kotlin?

easy

A. var name: String?

B. var name: ?String

C. var name: String!

D. var name: nullable String

Solution

1. Step 1: Recall Kotlin nullable syntax

   Kotlin uses ? after the type name to mark it nullable, e.g., String?.

2. Step 2: Check each option

   var name: String? uses String? correctly. Options A, B, and D use invalid syntax.

3. Final Answer:

   var name: String? -> Option A

4. Quick Check:

   Nullable syntax = type followed by ? [OK]

Hint: Put ? right after the type name for nullable [OK]

Common Mistakes:

• Placing ? before the type
• Using ! instead of ?
• Writing 'nullable' keyword explicitly

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

var text: String? = null
println(text?.length ?: "No length")

medium

A. 0

B. null

C. No length

D. Throws NullPointerException

Solution

1. Step 1: Understand safe call and Elvis operator

   text?.length safely accesses length if text is not null; else returns null. The Elvis operator ?: provides "No length" if left side is null.

2. Step 2: Evaluate given code

   Since text is null, text?.length is null, so the expression prints "No length".

3. Final Answer:

   No length -> Option C

4. Quick Check:

   Safe call + Elvis = fallback value [OK]

Hint: Safe call ?. returns null safely; ?: provides default [OK]

Common Mistakes:

• Expecting NullPointerException on null safe call
• Thinking null?.length returns 0
• Ignoring Elvis operator fallback

4. Identify the error in this Kotlin code snippet:

var number: Int? = 10
var result: Int = number + 5

medium

A. Variable 'result' should be nullable.

B. Variable 'number' cannot be nullable.

C. Missing semicolon after declaration.

D. Cannot add Int? and Int directly.

Solution

1. Step 1: Check type compatibility

   number is nullable Int?, but result expects non-null Int. Adding nullable and non-null directly is not allowed.

2. Step 2: Understand Kotlin's null safety

   You must safely unwrap number before adding, e.g., using number ?: 0 or safe call.

3. Final Answer:

   Cannot add Int? and Int directly. -> Option D

4. Quick Check:

   Nullable + non-null Int needs safe unwrap [OK]

Hint: Nullable Int can't be used directly in arithmetic [OK]

Common Mistakes:

• Assuming nullable Int can be used like Int
• Ignoring need for safe call or Elvis operator
• Thinking semicolon is required in Kotlin

5. You have a nullable list of nullable Strings: val items: List?. How do you safely print the length of the first item or print 0 if the list or first item is null?

hard

A. println(items?.get(0)?.length ?: 0)

B. println(items[0].length)

C. println(items!!.get(0)!!.length)

D. println(items?.get(0).length ?: 0)

Solution

1. Step 1: Analyze nullable types

   items can be null, and its elements can be null. So both list and first item need safe calls.

2. Step 2: Use safe calls and Elvis operator

   items?.get(0)?.length ?: 0 safely accesses first item length or returns 0 if null.

3. Final Answer:

   println(items?.get(0)?.length ?: 0) -> Option A

4. Quick Check:

   Safe calls chain + fallback = safe nullable access [OK]

Hint: Chain ?. for each nullable and use ?: for default [OK]

Common Mistakes:

• Using !! which throws exception if null
• Missing safe call on first item
• Assuming list or item can't be null