Val for immutable references in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an immutable variable named name with the value "Alice".

Kotlin

val name: String = [1]

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

A"Alice"

BAlice

Cvar Alice

Dval Alice

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the quotes around the string value.

Using var instead of val.

In Kotlin, val declares an immutable variable. The value must be assigned with quotes for a string, so "Alice" is correct.

2fill in blank

medium

Complete the code to declare an immutable integer variable age with the value 30.

Kotlin

val age: Int = [1]

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

A"30"

B30

Cage

Dval 30

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around numbers, making them strings.

Trying to declare the value with val again inside the assignment.

Integer values are written without quotes. So 30 is the correct value to assign.

3fill in blank

hard

Fix the error in the code by completing the declaration of an immutable list of strings named colors.

Kotlin

val colors: List<String> = [1]

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

AmutableListOf("red", "green", "blue")

BarrayOf("red", "green", "blue")

ClistOf("red", "green", "blue")

D["red", "green", "blue"]

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets like in other languages.

Using mutableListOf() which is mutable, not immutable.

To create an immutable list in Kotlin, use listOf(). arrayOf() creates an array, mutableListOf() creates a mutable list, and square brackets are not valid syntax.

4fill in blank

hard

Fill both blanks to declare an immutable variable pi of type Double with the value 3.14.

Kotlin

val [1]: [2] = 3.14

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

Api

Bradius

CDouble

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using Int type which cannot hold decimals.

Using a different variable name than pi.

The variable name is pi and the type is Double for decimal numbers.

5fill in blank

hard

Fill all three blanks to declare an immutable map named user with keys "name" and "age" and their respective values.

Kotlin

val [1] = mapOf([2] to "Bob", [3] to 25)

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

Auser

B"name"

C"age"

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting the keys as strings.

Using wrong variable names.

The map variable is named user. The keys are strings "name" and "age" mapped to their values.

Practice

1. What does the val keyword do in Kotlin?

easy

A. Defines a class

B. Creates an immutable reference that cannot be reassigned

C. Declares a function

D. Creates a mutable variable that can be reassigned

Solution

1. Step 1: Understand the role of val

   val declares a variable whose reference cannot be changed after assignment.

2. Step 2: Compare with other keywords

   var allows reassignment, but val does not.

3. Final Answer:

   Creates an immutable reference that cannot be reassigned -> Option B

4. Quick Check:

   val = immutable reference [OK]

Hint: Remember: val means fixed reference, var means changeable [OK]

Common Mistakes:

• Confusing val with var
• Thinking val makes the object itself immutable
• Assuming val declares a function or class

2. Which of the following is the correct syntax to declare an immutable integer variable named count with value 10 in Kotlin?

easy

A. val count = 10

B. var count = 10

C. int val count = 10

D. val count : Int =

Solution

1. Step 1: Identify correct keyword and syntax

   val is used for immutable variables, and Kotlin infers type if not specified.

2. Step 2: Check each option

   var count = 10 uses var (mutable), C has invalid syntax, D is incomplete.

3. Final Answer:

   val count = 10 -> Option A

4. Quick Check:

   Immutable variable syntax = val count = 10 [OK]

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

Common Mistakes:

• Using var instead of val for immutable variables
• Incorrect Kotlin type declarations
• Leaving declarations incomplete

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

val list = mutableListOf(1, 2, 3)
list.add(4)
println(list)

medium

A. Runtime error

B. [1, 2, 3]

C. Compilation error

D. [1, 2, 3, 4]

Solution

1. Step 1: Understand val with mutable objects

   val means the reference to the list cannot change, but the list itself can be modified.

2. Step 2: Analyze code behavior

   Adding 4 to the mutable list changes its content, so printing shows [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option D

4. Quick Check:

   val reference + mutable object = modified list [OK]

Hint: val fixes reference, not object content if mutable [OK]

Common Mistakes:

• Thinking val prevents modifying the list content
• Expecting compilation or runtime errors
• Confusing immutable reference with immutable object

4. Identify the error in the following Kotlin code:

val number = 5
number = 10
println(number)

medium

A. No error, prints 10

B. Runtime error: reassignment not allowed

C. Compilation error: val cannot be reassigned

D. Syntax error: missing semicolon

Solution

1. Step 1: Check reassignment of val variable

   val variables cannot be reassigned after initialization.

2. Step 2: Identify error type

   Trying to assign 10 to number causes a compilation error.

3. Final Answer:

   Compilation error: val cannot be reassigned -> Option C

4. Quick Check:

   val reassignment = compilation error [OK]

Hint: val variables cannot change after first assignment [OK]

Common Mistakes:

• Assuming val variables can be reassigned
• Confusing runtime errors with compile errors
• Ignoring Kotlin's no-semicolon rule

5. Given the following Kotlin code, what will be the output?

val map = mutableMapOf("a" to 1, "b" to 2)
map["a"] = 3
println(map)

Explain why this works even though map is declared with val.

hard

A. {a=3, b=2}

B. {a=1, b=2}

C. Compilation error: cannot modify val map

D. Runtime error

Solution

1. Step 1: Understand val with mutable collections

   val means the reference to the map cannot change, but the map's content can be modified.

2. Step 2: Analyze the map update

   Updating key "a" to 3 changes the map content, so printing shows {a=3, b=2}.

3. Final Answer:

   {a=3, b=2} -> Option A

4. Quick Check:

   val reference + mutable map = updated map content [OK]

Hint: val fixes reference, mutable map content can change [OK]

Common Mistakes:

• Thinking val prevents modifying map content
• Expecting compilation or runtime errors
• Confusing immutable reference with immutable object