Open classes and methods in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of overriding an open method

What is the output of this Kotlin code?

Kotlin

open class Animal {
    open fun sound() = "Some sound"
}

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

fun main() {
    val animal: Animal = Dog()
    println(animal.sound())
}

A Runtime error

B Bark

C Compilation error

D Some sound

💡 Hint

Remember that open methods can be overridden in subclasses.

The method sound() is open in Animal and overridden in Dog. The variable animal holds a Dog instance, so calling sound() prints "Bark".

❓ Predict Output

intermediate

Effect of missing open keyword on inheritance

What happens when you try to compile and run this Kotlin code?

Kotlin

class Vehicle {
    fun drive() = "Driving"
}

class Car : Vehicle() {
    override fun drive() = "Car driving"
}

fun main() {
    val car = Car()
    println(car.drive())
}

A Compilation error: 'drive' in 'Vehicle' is final and cannot be overridden

B Driving

C Car driving

D Runtime error

💡 Hint

Check if the method drive() is open for overriding.

In Kotlin, methods are final by default. Since drive() is not marked open, overriding it causes a compilation error.

🔧 Debug

advanced

Why does this override fail?

This Kotlin code fails to compile. What is the reason?

Kotlin

open class Parent {
    fun greet() = "Hello"
}

class Child : Parent() {
    override fun greet() = "Hi"
}

A Parent class must be abstract to allow overrides

B Child class must be open to override methods

C Missing override keyword causes error

D greet() in Parent is not open, so it cannot be overridden

💡 Hint

Check the declaration of greet() in Parent.

Only methods marked open can be overridden. greet() is final by default, so override causes error.

🧠 Conceptual

advanced

Open classes and inheritance behavior

Which statement about open classes and methods in Kotlin is true?

A Classes and methods are open by default, so they can be inherited and overridden without keywords

B Only classes are final by default; methods are open by default

C Classes and methods are final by default; you must mark them open to allow inheritance or overriding

D You cannot inherit from open classes

💡 Hint

Think about Kotlin's default behavior for classes and methods.

In Kotlin, classes and methods are final by default. You must explicitly mark them open to allow inheritance or overriding.

❓ Predict Output

expert

Output with open class and method with property override

What is the output of this Kotlin program?

Kotlin

open class Person {
    open val greeting: String = "Hello"
    open fun greet() = greeting
}

class Student : Person() {
    override val greeting: String = "Hi"
    override fun greet() = "${greeting}, student!"
}

fun main() {
    val p: Person = Student()
    println(p.greet())
}

A Hi, student!

B Compilation error

C Hello

D Hello, student!

💡 Hint

Remember that properties can be overridden and used in methods.

The Student class overrides greeting with "Hi" and greet() returns "Hi, student!". The variable p is a Person reference to a Student instance, so the overridden greet() runs.