Overriding methods with override in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of overridden method call

What is the output of this Kotlin code when calling greet() on an instance of Dog?

Kotlin

open class Animal {
    open fun greet() = "Hello from Animal"
}

class Dog : Animal() {
    override fun greet() = "Woof from Dog"
}

fun main() {
    val pet: Animal = Dog()
    println(pet.greet())
}

A Woof from Dog

B Hello from Animal

C Compilation error: missing override keyword

D Runtime error: method not found

💡 Hint

Remember that the override keyword allows a subclass to provide its own implementation of a method.

The Dog class overrides the greet() method from Animal. When calling greet() on a Dog instance, the overridden method in Dog runs, printing "Woof from Dog".

❓ Predict Output

intermediate

Output when calling base and overridden methods

What will be printed when running this Kotlin code?

Kotlin

open class Vehicle {
    open fun start() = "Vehicle started"
}

class Car : Vehicle() {
    override fun start() = "Car started"
    fun startBase() = super.start()
}

fun main() {
    val myCar = Car()
    println(myCar.start())
    println(myCar.startBase())
}

A Compilation error: cannot call super in this context

B Car started\nCar started

C Vehicle started\nVehicle started

D Car started\nVehicle started

💡 Hint

The super keyword calls the base class method even if it is overridden.

The start() method in Car overrides the base method and returns "Car started". The startBase() method calls super.start(), which runs the base class method returning "Vehicle started".

🔧 Debug

advanced

Identify the error in method overriding

Why does this Kotlin code fail to compile?

Kotlin

open class Shape {
    fun draw() = "Drawing shape"
}

class Circle : Shape() {
    override fun draw() = "Drawing circle"
}

A Cannot override 'draw' because it is not open in base class

B Missing override keyword in subclass

C Function signatures do not match

D No error, code compiles fine

💡 Hint

In Kotlin, only methods marked open can be overridden.

The draw() method in Shape is not marked open, so it cannot be overridden in Circle. This causes a compilation error.

📝 Syntax

advanced

Correct override syntax

Which option correctly overrides the calculate() method in Kotlin?

Kotlin

open class Calculator {
    open fun calculate(x: Int): Int = x * 2
}

class AdvancedCalculator : Calculator() {
    // override method here
}

A override calculate(x: Int): Int = x * 3

B fun override calculate(x: Int): Int = x * 3

C override fun calculate(x: Int): Int = x * 3

D override fun calculate(x): Int = x * 3

💡 Hint

The override keyword must come before fun and the parameter types must be specified.

Option C uses the correct syntax: override fun calculate(x: Int): Int = x * 3. Other options have syntax errors such as wrong keyword order, missing fun, or missing parameter types.

🚀 Application

expert

Determine output with multiple overrides and calls

What is the output of this Kotlin program?

Kotlin

open class A {
    open fun foo() = "A"
    fun bar() = foo()
}

class B : A() {
    override fun foo() = "B"
}

class C : B() {
    override fun foo() = "C"
    fun test() = super.foo()
}

fun main() {
    val c = C()
    println(c.foo())
    println(c.bar())
    println(c.test())
}

A C\nB\nB

B C\nC\nB

C B\nB\nA

D C\nA\nB

💡 Hint

Remember that super refers to the immediate superclass, and method calls use the most derived override.

Calling c.foo() runs C's override, printing "C". Calling c.bar() calls A's bar(), which calls foo() dynamically, so it runs C's foo(), printing "C". Calling c.test() calls super.foo() from C, which calls B's foo(), printing "B".