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.