Properties with val and var in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of val and var property modification

What is the output of this Kotlin code?

Kotlin

class Person {
    val name = "Alice"
    var age = 30
}

fun main() {
    val p = Person()
    // p.name = "Bob" // Uncommenting this line causes error
    p.age = 31
    println("Name: ${p.name}, Age: ${p.age}")
}

A Name: Alice, Age: 31

B Name: Bob, Age: 31

C Name: Alice, Age: 30

D Compilation error due to val property modification

💡 Hint

Remember val means read-only property, var means mutable property.

The val property 'name' cannot be reassigned, so the commented line would cause a compilation error if uncommented. The var property 'age' can be changed, so age becomes 31.

❓ Predict Output

intermediate

Property mutability in data classes

What will be the output of this Kotlin code?

Kotlin

data class Point(val x: Int, var y: Int)

fun main() {
    val p = Point(10, 20)
    // p.x = 15 // Uncommenting causes error
    p.y = 25
    println("x = ${p.x}, y = ${p.y}")
}

A Compilation error due to val property reassignment

B x = 15, y = 25

C x = 10, y = 25

D x = 10, y = 20

💡 Hint

val properties cannot be reassigned, var properties can.

The val property 'x' is immutable and cannot be changed after initialization. The var property 'y' can be changed, so it becomes 25.

🔧 Debug

advanced

Identify the error with val property reassignment

What error does this Kotlin code produce?

Kotlin

class Car {
    val model = "Sedan"
}

fun main() {
    val c = Car()
    c.model = "SUV"
    println(c.model)
}

A Runtime error: Uninitialized property access

B Compilation error: Val cannot be reassigned

C Compilation error: Variable 'c' must be mutable

D No error, prints: SUV

💡 Hint

val properties cannot be reassigned after initialization.

The property 'model' is declared with val, so it cannot be reassigned. Trying to assign a new value causes a compilation error.

🧠 Conceptual

advanced

Difference between val and var in Kotlin properties

Which statement correctly describes the difference between val and var properties in Kotlin?

A val properties are immutable and cannot be reassigned; var properties are mutable and can be reassigned.

B val properties can be reassigned only inside the class; var properties cannot be reassigned.

C val properties are mutable only if declared inside a data class; var properties are always immutable.

D val and var properties behave the same; the difference is only in naming convention.

💡 Hint

Think about whether you can change the value after it is set.

val means the property is read-only after initialization, var means it can be changed anytime.

❓ Predict Output

expert

Output with val and var in inheritance

What is the output of this Kotlin code?

Kotlin

open class Animal {
    open val sound: String = ""
}

class Dog : Animal() {
    override val sound = "Bark"
}

class Cat : Animal() {
    override var sound = "Meow"
}

fun main() {
    val dog = Dog()
    val cat = Cat()
    println("Dog sound: ${dog.sound}")
    println("Cat sound: ${cat.sound}")
    cat.sound = "Purr"
    println("Cat new sound: ${cat.sound}")
}

A

Dog sound: 
Cat sound: Meow
Cat new sound: Purr

B

Dog sound: Bark
Cat sound: Meow
Compilation error on cat.sound reassignment

C

Dog sound: Bark
Cat sound: Meow
Cat new sound: Purr

D Compilation error: Cannot override val with var

💡 Hint

val can be overridden by val, var can be overridden by var, but val cannot be overridden by var and vice versa.

Dog overrides val with val, which is allowed. Cat attempts to override the open val 'sound' with var, which is not allowed in Kotlin because it violates the immutability contract of the base class property. This causes a compilation error.