Non-nullable types by default in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a non-nullable integer variable named age with value 30.

Kotlin

val age: Int = [1]

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

A30

Bnull

C"30"

D30L

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning null to a non-nullable type causes a compilation error.

Using quotes around numbers makes them strings, not integers.

The variable age is declared as a non-nullable Int, so it must be assigned a non-null integer value like 30.

2fill in blank

medium

Complete the code to declare a non-nullable string variable name with value "Alice".

Kotlin

val name: String = [1]

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

Anull

BAlice

C"Alice"

Dalice

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around string literals.

Assigning null to a non-nullable variable.

Strings must be enclosed in double quotes. Since name is non-nullable, it cannot be assigned null.

3fill in blank

hard

Fix the error in the code by completing the declaration of a non-nullable Boolean variable isActive with value true.

Kotlin

val isActive: Boolean = [1]

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

Atrue

Bnull

C"true"

DTrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase True instead of lowercase true.

Assigning null to a non-nullable variable.

Using quotes around Boolean values.

The Boolean value true is lowercase in Kotlin and must not be in quotes. Non-nullable variables cannot be assigned null.

4fill in blank

hard

Fill both blanks to declare a non-nullable variable height of type Double with value 1.75.

Kotlin

val height: [1] = [2]

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

ADouble

B1.75

CFloat

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using Float instead of Double when the type is specified as Double.

Assigning null to a non-nullable variable.

The variable height is declared as a non-nullable Double type and assigned the value 1.75.

5fill in blank

hard

Fill all three blanks to create a non-nullable variable score of type Int, assigned the value 100, and print it.

Kotlin

val [1]: [2] = [3]
println(score)

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

Ascore

BInt

C100

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using null as value for a non-nullable variable.

Mixing up the order of variable name, type, and value.

This code declares a non-nullable integer variable score with value 100 and prints it.

Practice

1. In Kotlin, what happens if you declare a variable without a ? after its type?

easy

A. The variable cannot hold null values.

B. The variable can hold null values.

C. The variable is automatically initialized to null.

D. The variable must be declared as lateinit.

Solution

1. Step 1: Understand Kotlin's default type behavior

   Kotlin types are non-nullable by default, meaning variables cannot hold null unless specified.

2. Step 2: Check the effect of missing ?

   Without ?, the variable type is non-nullable, so it cannot hold null.

3. Final Answer:

   The variable cannot hold null values. -> Option A

4. Quick Check:

   Non-nullable type = no null allowed [OK]

Hint: No ? means no null allowed [OK]

Common Mistakes:

• Thinking variables are nullable by default
• Assuming variables initialize to null automatically
• Confusing lateinit with nullability

2. Which of the following Kotlin variable declarations is correct to allow null values?

easy

A. val name: String? = null

B. val name: String?

C. val name: String = null

D. val name: String = "null"

Solution

1. Step 1: Identify nullable type syntax

   Adding ? after the type allows the variable to hold null.

2. Step 2: Check initialization

   Nullable variables must be initialized or declared with null explicitly, so val name: String? = null is correct.

3. Final Answer:

   val name: String? = null -> Option A

4. Quick Check:

   Nullable type with ? and initialized to null [OK]

Hint: Use ? and initialize nullable variables [OK]

Common Mistakes:

• Declaring nullable without ?
• Not initializing nullable variables
• Assigning null to non-nullable types

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

fun main() {
    val text: String? = null
    println(text?.length ?: "No text")
}

medium

A. 0

B. Compilation error

C. null

D. No text

Solution

1. Step 1: Understand safe call operator usage

   The text?.length safely accesses length only if text is not null.

2. Step 2: Evaluate the Elvis operator

   Since text is null, text?.length is null, so the Elvis operator ?: returns "No text".

3. Final Answer:

   No text -> Option D

4. Quick Check:

   Safe call + Elvis operator returns fallback [OK]

Hint: Safe call with Elvis returns fallback if null [OK]

Common Mistakes:

• Expecting 0 instead of fallback string
• Confusing safe call with normal call
• Thinking code causes error due to null

4. Identify the error in this Kotlin code snippet:

fun main() {
    val number: Int = null
    println(number)
}

medium

A. Variable 'number' is not initialized.

B. Cannot assign null to a non-nullable type Int.

C. Missing semicolon after println statement.

D. Variable 'number' should be declared as var.

Solution

1. Step 1: Check variable type and assignment

   The variable 'number' is declared as non-nullable Int but assigned null, which is not allowed.

2. Step 2: Understand Kotlin null safety rules

   Non-nullable types cannot hold null, so this causes a compilation error.

3. Final Answer:

   Cannot assign null to a non-nullable type Int. -> Option B

4. Quick Check:

   Non-nullable types reject null assignments [OK]

Hint: Non-nullable types cannot be assigned null [OK]

Common Mistakes:

• Thinking semicolon is required
• Assuming var fixes null assignment
• Ignoring Kotlin's null safety

5. You have a nullable variable val input: String?. You want to print its length only if it is not null, otherwise print "Empty input". Which Kotlin code correctly does this?

hard

A. println(input.length ?: "Empty input")

B. println(input!!.length)

C. println(if (input != null) input.length else "Empty input")

D. println(input?.length)

Solution

1. Step 1: Understand nullable variable usage

   Since input can be null, we must check before accessing length.

2. Step 2: Evaluate each option

   println(input.length ?: "Empty input") is invalid because length is Int, not nullable, so Elvis operator on it is wrong.
   println(if (input != null) input.length else "Empty input") safely checks null and prints length or fallback.
   println(input!!.length) uses !! which throws exception if null.
   println(input?.length) prints null if input is null, not the string "Empty input".

3. Final Answer:

   println(if (input != null) input.length else "Empty input") -> Option C

4. Quick Check:

   Explicit null check with if-else for safe access [OK]

Hint: Use if-else to check null before accessing properties [OK]

Common Mistakes:

• Using !! and risking exceptions
• Misusing Elvis operator on non-nullable Int
• Printing null instead of fallback string