Variables (val, var) and null safety in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable that can be changed later.

Android Kotlin

var name: String = "John"
name = [1]

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

A"Alice"

Bval

Cvar

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using val instead of a string value to assign.

Trying to assign null without making the variable nullable.

We use var to declare a variable that can change. Assigning "Alice" updates the value.

2fill in blank

medium

Complete the code to declare a constant string that cannot be changed.

Android Kotlin

val greeting: String = [1]

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

Anull

Bval

Cvar

D"Hello"

Attempts:

3 left

💡 Hint

Common Mistakes

Using var instead of val for constants.

Assigning null without nullable type.

val declares a constant. Assigning "Hello" sets the constant string value.

3fill in blank

hard

Fix the error in the code by making the variable nullable.

Android Kotlin

var age: Int = [1]
age = null

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

AInt?

B0

Cnull

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign null to a non-nullable type.

Using var keyword inside the assignment.

Adding ? after Int makes the variable nullable, so it can hold null.

4fill in blank

hard

Fill both blanks to declare a nullable variable and assign null to it.

Android Kotlin

var city: [1] = [2]

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

AString?

Bnull

CString

D"Paris"

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring variable as non-nullable String and assigning null.

Assigning a string value instead of null.

String? makes city nullable. Assigning null is allowed then.

5fill in blank

hard

Fill all three blanks to safely print the length of a nullable string.

Android Kotlin

val length = name[1][2][3]

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

A?

B.length

C?: 0

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Using !! which throws error if null.

Not providing default value for null case.

The safe call operator ? allows access if not null, .length gets length, ?: 0 provides default if null.

Practice

1. Which keyword should you use in Kotlin for a variable whose value will never change after assignment?

easy

A. let

B. var

C. val

D. const

Solution

1. Step 1: Understand Kotlin variable keywords

   val is used for read-only variables, meaning their value cannot change once assigned.

2. Step 2: Compare with other keywords

   var allows changing the value; const is not a Kotlin keyword for variables; let is from other languages like Swift or JavaScript.

3. Final Answer:

   val -> Option C

4. Quick Check:

   Immutable variable = val [OK]

Hint: Use val for fixed values, var for changeable ones [OK]

Common Mistakes:

• Confusing val and var
• Using const which is not Kotlin variable keyword
• Thinking let is Kotlin keyword

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

easy

A. var name: String? = null

B. var name: String?

C. var name: String = null

D. var name: String = "null"

Solution

1. Step 1: Understand nullable type syntax

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

2. Step 2: Check initialization

   Nullable variables should be initialized with null if no value is assigned yet.

3. Final Answer:

   var name: String? = null -> Option A

4. Quick Check:

   Nullable variable syntax = var name: String? = null [OK]

Hint: Use ? after type and assign null to allow null values [OK]

Common Mistakes:

• Assigning null to non-nullable type
• Forgetting ? for nullable types
• Using string "null" instead of null

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

var count: Int? = 5
count = null
println(count)

medium

A. null

B. 0

C. 5

D. Compilation error

Solution

1. Step 1: Analyze variable declaration

   count is declared as nullable Int (Int?) and initially set to 5.

2. Step 2: Assign null and print

   Then count is assigned null, which is allowed for nullable types, so printing it outputs null.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Nullable variable assigned null prints null [OK]

Hint: Nullable variables can hold null and print it directly [OK]

Common Mistakes:

• Expecting 5 after reassignment
• Thinking null prints 0
• Assuming compilation error for null assignment

4. Identify the error in this Kotlin code snippet:

val message: String = null
println(message)

medium

A. No error, code runs fine

B. val cannot be used with String type

C. println cannot print val variables

D. Cannot assign null to non-nullable type String

Solution

1. Step 1: Check variable type and assignment

   message is declared as non-nullable String but assigned null, which is not allowed.

2. Step 2: Understand Kotlin null safety rules

   Non-nullable types cannot hold null; to allow null, type must be String?.

3. Final Answer:

   Cannot assign null to non-nullable type String -> Option D

4. Quick Check:

   Non-nullable String cannot be null [OK]

Hint: Non-nullable types cannot hold null values [OK]

Common Mistakes:

• Thinking val disallows null but var allows
• Assuming println can't print val
• Ignoring Kotlin's null safety rules

5. You want to create a variable that can change its value but sometimes hold no value at all. Which declaration is correct in Kotlin?

hard

A. val data: Int = null

B. var data: Int? = null

C. val data: Int? = 0

D. var data: Int = null

Solution

1. Step 1: Understand mutability and nullability

   The variable must be changeable, so var is needed, and it must allow null, so type must be nullable with ?.

2. Step 2: Check each option

   val data: Int = null uses val (immutable) and assigns null to non-nullable type (error). var data: Int? = null uses var and nullable type with null initial value (correct). val data: Int? = 0 uses val (immutable) but nullable type with 0 initial value (not changeable). var data: Int = null uses var but non-nullable type assigned null (error).

3. Final Answer:

   var data: Int? = null -> Option B

4. Quick Check:

   Mutable and nullable = var with ? [OK]

Hint: Use var for changeable and ? for nullable types [OK]

Common Mistakes:

• Using val when variable should change
• Assigning null to non-nullable type
• Confusing val and var usage