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.

Practice

1. What is the main difference between val and var properties in Kotlin?

easy

A. val can be changed anytime, var is read-only after initialization.

B. val is read-only after initialization, var can be changed anytime.

C. val and var both can be changed anytime.

D. val and var are exactly the same.

Solution

1. Step 1: Understand val behavior

   val properties are assigned once and cannot be reassigned later.

2. Step 2: Understand var behavior

   var properties can be reassigned multiple times after initialization.

3. Final Answer:

   val is read-only after initialization, var can be changed anytime. -> Option B

4. Quick Check:

   val = read-only, var = mutable [OK]

Hint: Remember: val = fixed, var = variable [OK]

Common Mistakes:

• Confusing val as mutable
• Thinking var is read-only
• Believing val and var behave the same

2. Which of the following is the correct way to declare a mutable property named age of type Int in Kotlin?

easy

A. var age: Int = 25

B. val age: Int = 25

C. val age = 25

D. var age: Int

Solution

1. Step 1: Identify mutable property keyword

   Mutable properties use var keyword.

2. Step 2: Check initialization

   Property must be initialized or declared nullable; here age is initialized with 25.

3. Final Answer:

   var age: Int = 25 -> Option A

4. Quick Check:

   Mutable property uses var [OK]

Hint: Use var for changeable properties [OK]

Common Mistakes:

• Using val for mutable property
• Not initializing non-nullable property
• Omitting type without initialization

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

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

fun main() {
    val p = Person()
    println(p.name)
    p.age = 31
    println(p.age)
}

medium

A. Anna
30

B. Anna
null

C. Error: Cannot assign to val property

D. Anna
31

Solution

1. Step 1: Check val name usage

   name is a val, so it prints "Anna" and cannot be changed.

2. Step 2: Check var age reassignment

   age is a var, so it can be changed from 30 to 31 before printing.

3. Final Answer:

   Anna 31 -> Option D

4. Quick Check:

   val prints original, var prints updated [OK]

Hint: val prints original, var can be updated [OK]

Common Mistakes:

• Expecting error on changing var
• Thinking val can be reassigned
• Confusing printed values

4. Identify the error in the following Kotlin code:

class Car {
    val model: String
    var year: Int

    init {
        model = "Tesla"
        year = 2020
    }
}

fun main() {
    val car = Car()
    car.model = "BMW"
    println(car.model)
}

medium

A. Cannot assign a new value to val property 'model'.

B. Property 'year' is not initialized.

C. Missing constructor parameters.

D. No error, code runs fine.

Solution

1. Step 1: Check val model reassignment

   model is declared as val, so it cannot be reassigned after initialization.

2. Step 2: Verify property initialization

   model and year are both initialized in init, so no initialization error.

3. Final Answer:

   Cannot assign a new value to val property 'model'. -> Option A

4. Quick Check:

   val properties cannot be reassigned [OK]

Hint: val properties cannot be changed after init [OK]

Common Mistakes:

• Trying to reassign val property
• Ignoring uninitialized var warnings
• Assuming val can be reassigned

5. You want to create a Kotlin class Book where the title should never change after creation, but the price can be updated. Which declaration is correct?

hard

A. class Book(val title: String, val price: Double)

B. class Book(var title: String, var price: Double)

C. class Book(val title: String, var price: Double)

D. class Book(var title: String, val price: Double)

Solution

1. Step 1: Determine immutability for title

   title should not change, so declare it with val.

2. Step 2: Determine mutability for price

   price can change, so declare it with var.

3. Final Answer:

   class Book(val title: String, var price: Double) -> Option C

4. Quick Check:

   Immutable title = val, mutable price = var [OK]

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

Common Mistakes:

• Using var for title which should be fixed
• Using val for price which should be changeable
• Mixing up val and var keywords