Var for mutable references in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using var?

Consider the following Kotlin code snippet. What will be printed?

Kotlin

fun main() {
    var number = 10
    number = 20
    println(number)
}

A 20

B 10

C Compilation error

D Runtime exception

💡 Hint

Remember that var allows the variable to be changed.

The variable number is declared with var, so it can be reassigned. Initially it is 10, then changed to 20, so println outputs 20.

❓ Predict Output

intermediate

What happens when you reassign a val variable?

What will happen when running this Kotlin code?

Kotlin

fun main() {
    val text = "Hello"
    text = "World"
    println(text)
}

A World

B Runtime exception

C Hello

D Compilation error

💡 Hint

val means the variable cannot be reassigned.

The variable text is declared with val, which means it is read-only. Trying to assign a new value causes a compilation error.

❓ Predict Output

advanced

What is the output when modifying a mutable list declared with var?

Analyze this Kotlin code and determine the output.

Kotlin

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

A [1, 2, 3, 4]

B Runtime exception

C [1, 2, 3]

D Compilation error

💡 Hint

Mutable lists can be changed even if declared with var.

The variable list is mutable and declared with var. Adding 4 modifies the list, so printing it shows all four elements.

❓ Predict Output

advanced

What error occurs when trying to reassign a val mutable list?

What will happen when running this Kotlin code?

Kotlin

fun main() {
    val list = mutableListOf(1, 2, 3)
    list = mutableListOf(4, 5, 6)
    println(list)
}

A [4, 5, 6]

B [1, 2, 3]

C Compilation error

D Runtime exception

💡 Hint

val means the reference cannot be changed, even if the object is mutable.

The variable list is declared with val, so you cannot assign a new list to it. This causes a compilation error.

🧠 Conceptual

expert

Why use var for mutable references in Kotlin?

Which statement best explains why var is used for mutable references in Kotlin?

A <p>Because <code>var</code> makes the object itself immutable.</p>

B <p>Because <code>var</code> allows changing the reference to point to a different object or value.</p>

C <p>Because <code>var</code> prevents any changes to the variable after initialization.</p>

D <p>Because <code>var</code> automatically synchronizes access in multithreaded code.</p>

💡 Hint

Think about what var allows you to do with a variable.

var means the variable's reference can be changed to point to a different object or value. It does not make the object immutable or handle synchronization.