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".

Practice

1. What keyword must be used in Kotlin to allow a method in a parent class to be overridden in a child class?

easy

A. open

B. override

C. final

D. abstract

Solution

1. Step 1: Understand method overriding rules in Kotlin

   In Kotlin, methods are final by default and cannot be overridden unless marked otherwise.

2. Step 2: Identify the keyword to allow overriding

   The open keyword is used in the parent class method to allow it to be overridden in subclasses.

3. Final Answer:

   open -> Option A

4. Quick Check:

   Parent method must be open to override [OK]

Hint: Parent method needs 'open' to allow override [OK]

Common Mistakes:

• Using 'override' in parent method instead of 'open'
• Forgetting to mark parent method as 'open'
• Using 'final' which prevents overriding

2. Which of the following is the correct way to override a method named greet in a Kotlin subclass?

easy

A. fun greet() { println("Hello") }

B. override fun greet() { println("Hello") }

C. open fun greet() { println("Hello") }

D. final fun greet() { println("Hello") }

Solution

1. Step 1: Recall syntax for overriding methods in Kotlin

   To override a method, the subclass method must use the override keyword.

2. Step 2: Check each option

   override fun greet() { println("Hello") } correctly uses override fun greet(). fun greet() { println("Hello") } misses override. open fun greet() { println("Hello") } uses open which is for parent methods. final fun greet() { println("Hello") } uses final which prevents overriding.

3. Final Answer:

   override fun greet() { println("Hello") } -> Option B

4. Quick Check:

   Use override keyword in subclass method [OK]

Hint: Use 'override' keyword before method in subclass [OK]

Common Mistakes:

• Omitting 'override' keyword in subclass method
• Using 'open' instead of 'override' in subclass
• Marking subclass method as 'final' which disallows override

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

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

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

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

medium

A. Generic sound

B. Compilation error

C. Bark

D. Runtime exception

Solution

1. Step 1: Understand method overriding and polymorphism

   The variable pet is of type Animal but holds a Dog object. The overridden sound() method in Dog will be called.

2. Step 2: Determine output of pet.sound()

   Since Dog overrides sound() to return "Bark", the output will be "Bark".

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Overridden method runs based on actual object type [OK]

Hint: Overridden method runs from actual object type [OK]

Common Mistakes:

• Thinking output is from parent class method
• Confusing variable type with object type
• Expecting compilation or runtime errors

4. Identify the error in the following Kotlin code:

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

class Car : Vehicle() {
    override fun start() = "Car starting"
}

medium

A. No error, code is correct

B. Missing 'override' keyword in Car's start method

C. Cannot override a method in Kotlin

D. Missing 'open' keyword on Vehicle's start method

Solution

1. Step 1: Check if parent method allows overriding

   The start() method in Vehicle is not marked open, so it cannot be overridden.

2. Step 2: Identify the error in subclass

   The Car class tries to override start() but parent method is not open, causing a compilation error.

3. Final Answer:

   Missing 'open' keyword on Vehicle's start method -> Option D

4. Quick Check:

   Parent method must be 'open' to override [OK]

Hint: Parent method must be 'open' to override it [OK]

Common Mistakes:

• Forgetting to mark parent method as 'open'
• Trying to override non-open method
• Assuming 'override' alone allows overriding

5. Given the following Kotlin classes, what will be the output when main() is run?

open class Printer {
    open fun print() = "Printing document"
}

class ColorPrinter : Printer() {
    override fun print() = "Printing in color"
}

class PhotoPrinter : ColorPrinter() {
    override fun print() = "Printing photo in color"
}

fun main() {
    val printer: Printer = PhotoPrinter()
    println(printer.print())
}

hard

A. Printing photo in color

B. Printing in color

C. Printing document

D. Compilation error due to multiple overrides

Solution

1. Step 1: Understand class hierarchy and method overriding

   PhotoPrinter overrides print() from ColorPrinter, which overrides Printer's method.

2. Step 2: Determine which method runs for printer.print()

   The variable printer is of type Printer but holds a PhotoPrinter object, so the most derived override runs, which is PhotoPrinter.print().

3. Final Answer:

   Printing photo in color -> Option A

4. Quick Check:

   Most derived override runs based on actual object type [OK]

Hint: Most derived override runs, even with parent type variable [OK]

Common Mistakes:

• Thinking parent class method runs instead of child's
• Confusing override chain in multi-level inheritance
• Expecting compilation error due to multiple overrides