Val for immutable references in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of val with mutable object

What is the output of this Kotlin code snippet?

Kotlin

data class Person(var name: String)

fun main() {
    val person = Person("Alice")
    person.name = "Bob"
    println(person.name)
}

A Alice

B Runtime exception

C Compilation error

D Bob

💡 Hint

Remember val means the reference cannot change, but the object it points to can be modified if mutable.

The val keyword makes the reference immutable, but the object itself can be changed if it has mutable properties. Here, person.name is changed from "Alice" to "Bob".

❓ Predict Output

intermediate

Reassigning val variable

What happens when you try to reassign a val variable in Kotlin?

Kotlin

fun main() {
    val number = 10
    // number = 20
    println(number)
}

A Compilation error if reassignment uncommented

B Prints 10

C Prints 20

D Runtime exception

💡 Hint

val variables cannot be reassigned after initialization.

Trying to assign a new value to a val variable causes a compilation error because val means the reference is immutable.

🔧 Debug

advanced

Why does this code fail to compile?

Identify the reason this Kotlin code does not compile.

Kotlin

fun main() {
    val list = listOf(1, 2, 3)
    list.add(4)
    println(list)
}

A val variables cannot call functions

B listOf creates a mutable list but val forbids modification

C list is immutable, so add() is not available

D println cannot print lists

💡 Hint

Check the type returned by listOf and what methods it supports.

listOf returns an immutable list. The add() method is not available on immutable lists, so the code fails to compile.

🧠 Conceptual

advanced

Val and object mutability

Which statement best describes val in Kotlin regarding object mutability?

A val makes both the reference and the object immutable

B val makes the reference immutable but the object can be mutable

C val allows reassigning the reference but not modifying the object

D val allows both reference reassignment and object mutation

💡 Hint

Think about what val restricts: the reference or the object?

val means the reference cannot be changed to point to another object, but the object itself can be mutable and changed.

❓ Predict Output

expert

Output with val and mutable collection

What is the output of this Kotlin program?

Kotlin

fun main() {
    val numbers = mutableListOf(1, 2, 3)
    val alias = numbers
    alias.add(4)
    println(numbers)
}

A [1, 2, 3, 4]

B Compilation error due to val reassignment

C Runtime exception

D [1, 2, 3]

💡 Hint

Both val variables point to the same mutable list object.

Both numbers and alias refer to the same mutable list. Adding 4 via alias modifies the list, so printing numbers shows the updated list.

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