Overriding methods with override in Kotlin - Mini Project: Build & Apply

Overriding methods with override

📖 Scenario: Imagine you are creating a simple app that manages different types of vehicles. Each vehicle can describe itself with a message. You will learn how to change this message for specific vehicle types by overriding a method.

🎯 Goal: Build a Kotlin program that defines a base class Vehicle with a method describe(). Then create a subclass Car that overrides the describe() method to provide a custom message. Finally, print the description from both classes.

📋 What You'll Learn

Create a base class called Vehicle with a method describe() that returns the string "This is a vehicle."

Create a subclass called Car that inherits from Vehicle

Override the describe() method in Car to return the string "This is a car." using the override keyword

Create instances of Vehicle and Car and print the result of calling describe() on each

💡 Why This Matters

🌍 Real World

Overriding methods is common when you want different types of objects to behave differently while sharing a common interface, like different vehicles describing themselves uniquely.

💼 Career

Understanding method overriding is essential for Kotlin developers working on apps that use inheritance and polymorphism to organize code cleanly and reuse functionality.

1

Create the base class Vehicle

Create a class called Vehicle with a method describe() that returns the string "This is a vehicle."

Kotlin

class Vehicle {
    // Your code here
}

Hint

Define a class named Vehicle. Inside it, write a function describe() that returns the exact string.

2

Create the subclass Car

Create a class called Car that inherits from Vehicle

Kotlin

class Vehicle {
    fun describe(): String {
        return "This is a vehicle."
    }
}

// Your code here

Hint

Use class Car : Vehicle() to make Car inherit from Vehicle.

3

Override the describe() method in Car

Inside the Car class, override the describe() method using the override keyword to return the string "This is a car."

Kotlin

open class Vehicle {
    fun describe(): String {
        return "This is a vehicle."
    }
}

class Car : Vehicle() {
    // Your code here
}

Hint

Remember to mark the base class method as open and use override in the subclass method.

4

Create instances and print descriptions

Create an instance of Vehicle called vehicle and an instance of Car called car. Then print the result of calling describe() on both instances.

Kotlin

open class Vehicle {
    open fun describe(): String {
        return "This is a vehicle."
    }
}

class Car : Vehicle() {
    override fun describe(): String {
        return "This is a car."
    }
}

// Your code here

Hint

Create variables vehicle and car with Vehicle() and Car(). Use println() to show their descriptions.

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