Why immutability by default matters in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an immutable variable in Kotlin.

Kotlin

val number = [1]

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

Avar

B10

Cmutable

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' instead of a value for assignment.

Trying to assign 'val' as a value.

In Kotlin, val declares an immutable variable, and number is assigned the value 10.

2fill in blank

medium

Complete the code to declare a mutable variable in Kotlin.

Kotlin

var count = [1]

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

A5

Bval

Cvar

Dimmutable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' as the value instead of a number.

Using 'val' as the value.

The variable count is mutable and initialized with the value 5.

3fill in blank

hard

Fix the error in the code by choosing the correct keyword for immutability.

Kotlin

[1] message = "Hello"

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

Avar

Bmutable

Cconst

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' which allows the variable to be changed.

Using 'const' which is not valid here.

val declares an immutable variable, which is the correct keyword here.

4fill in blank

hard

Fill both blanks to create a list that cannot be changed and a list that can be changed.

Kotlin

val immutableList = listOf(1, 2, 3)
[2] mutableList = [1](1, 2, 3)

mutableList = listOf(4, 5, 6)

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

AmutableListOf

Bval

Cvar

DlistOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using listOf for mutable list.

Using val for mutable list variable.

mutableListOf creates a mutable list, and var allows reassignment of mutableList.

5fill in blank

hard

Fill all three blanks to create a map with immutable keys and values, filtering only entries with values greater than 10.

Kotlin

val data = mapOf("a" to 5, "b" to 15, "c" to 20)
val filtered = data.filter { ([1], [2]) -> [3] > 10 }

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

Akey

Bvalue

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for key and value.

Using incorrect variable names that don't match the lambda destructuring.

The lambda destructures the map entry into k and value, then filters entries where value is greater than 10.

Practice

1. Why does Kotlin use val for variables by default instead of var?

easy

A. Because val creates immutable variables, making code safer and easier to understand.

B. Because val allows variables to change values anytime.

C. Because var is deprecated in Kotlin.

D. Because val is faster to execute than var.

Solution

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

   Kotlin uses val by default to create immutable variables that cannot be changed after assignment.

2. Step 2: Recognize benefits of immutability

   Immutable variables prevent accidental changes, making code safer and easier to read and maintain.

3. Final Answer:

   Because val creates immutable variables, making code safer and easier to understand. -> Option A

4. Quick Check:

   Immutability = safer code [OK]

Hint: Remember: val means fixed value, var means variable value [OK]

Common Mistakes:

• Thinking val allows value changes
• Believing var is deprecated
• Assuming val is faster than var

2. Which of the following Kotlin declarations correctly creates an immutable variable?

easy

A. var name = "Alice"

B. val name = "Alice"

C. immutable name = "Alice"

D. const var name = "Alice"

Solution

1. Step 1: Identify Kotlin syntax for immutable variables

   Kotlin uses val to declare immutable variables that cannot be reassigned.

2. Step 2: Check each option

   val name = "Alice" uses val correctly. var name = "Alice" uses var which is mutable. The other two options use invalid syntax.

3. Final Answer:

   val name = "Alice" -> Option B

4. Quick Check:

   Immutable variable syntax = val [OK]

Hint: val means immutable, var means mutable [OK]

Common Mistakes:

• Using var instead of val for immutability
• Trying to use 'immutable' keyword
• Combining const and var incorrectly

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

val x = 10
// x = 20
println(x)

medium

A. 20

B. Runtime error

C. 10

D. Compilation error

Solution

1. Step 1: Understand the code behavior

   The variable x is declared with val, so it is immutable and cannot be reassigned.

2. Step 2: Analyze the commented reassignment

   The reassignment x = 20 is commented out, so it does not run. The value of x remains 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   Immutable val keeps value 10 [OK]

Hint: Commented code does not run, so value stays same [OK]

Common Mistakes:

• Assuming commented code runs
• Thinking val variables can be reassigned
• Confusing compilation and runtime errors

4. Identify the error in this Kotlin code:

val count = 5
count = 10
println(count)

medium

A. Syntax error: missing semicolon

B. No error, prints 10

C. Runtime error: variable not initialized

D. Compilation error: val cannot be reassigned

Solution

1. Step 1: Check variable declaration

   count is declared with val, making it immutable.

2. Step 2: Analyze reassignment

   Trying to assign count = 10 causes a compilation error because val variables cannot be reassigned.

3. Final Answer:

   Compilation error: val cannot be reassigned -> Option D

4. Quick Check:

   val reassignment = compilation error [OK]

Hint: val variables cannot be reassigned after declaration [OK]

Common Mistakes:

• Thinking val variables can be reassigned
• Confusing runtime and compilation errors
• Assuming semicolons are required in Kotlin

5. You want to create a list of user names that should never change after creation. Which Kotlin declaration is best and why?

val users = listOf("Anna", "Bob", "Cara")

hard

A. Use val with listOf because it creates an immutable list and reference.

B. Use val with mutableListOf to allow adding users later.

C. Use var with listOf because the list can be changed later.

D. Use var with mutableListOf to change the list and reference.

Solution

1. Step 1: Understand immutability of variable and list

   val means the reference cannot change, and listOf creates an immutable list that cannot be modified.

2. Step 2: Compare options

   Use val with listOf because it creates an immutable list and reference. uses val and listOf, ensuring both the reference and list contents cannot change, which matches the requirement.

3. Final Answer:

   Use val with listOf because it creates an immutable list and reference. -> Option A

4. Quick Check:

   val + listOf = fully immutable list [OK]

Hint: val + listOf = fixed list and reference [OK]

Common Mistakes:

• Using var with mutableListOf when immutability is needed
• Confusing mutableListOf with listOf
• Thinking val allows list content changes