Overriding methods with override in Kotlin

Introduction

Overriding lets you change how a method works in a child class. It helps you customize behavior while keeping the same method name.

When you want a child class to do something different from its parent class for the same action.

When you need to add extra steps before or after the original method runs.

When you want to fix or improve how a method works in a subclass.

When you want to provide specific behavior for different types of objects in a class hierarchy.

Syntax

Kotlin

open class Parent {
    open fun greet() {
        println("Hello from Parent")
    }
}

class Child : Parent() {
    override fun greet() {
        println("Hello from Child")
    }
}

The open keyword allows a method to be overridden.

The override keyword tells Kotlin you are changing the parent's method.

Examples

Here, Dog changes the sound method to print "Bark" instead of "Some sound".

Kotlin

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

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

Car overrides start to give a specific message.

Kotlin

open class Vehicle {
    open fun start() {
        println("Vehicle starting")
    }
}

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

Sample Program

This program shows how Student changes the introduce method from Person. When run, it prints different greetings.

Kotlin

open class Person {
    open fun introduce() {
        println("Hi, I am a person.")
    }
}

class Student : Person() {
    override fun introduce() {
        println("Hi, I am a student.")
    }
}

fun main() {
    val person = Person()
    val student = Student()
    person.introduce()
    student.introduce()
}

Important Notes

You must mark the parent method with open to allow overriding.

Use override in the child method to avoid mistakes and make your code clear.

If you forget override, Kotlin will give an error.

Summary

Use open on parent methods to allow overriding.

Use override in child classes to change method behavior.

Overriding helps customize how objects behave while keeping the same method names.

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