Open classes and methods in Kotlin - Mini Project: Build & Apply

Open Classes and Methods in Kotlin

📖 Scenario: You are creating a simple app that manages vehicles. You want to allow future changes by letting other developers add new types of vehicles or change how existing ones behave.

🎯 Goal: Learn how to make a class and its method open for extension in Kotlin. You will create an open class, then override its method in a subclass, and finally print the results.

📋 What You'll Learn

Create an open class named Vehicle with an open method drive() that returns a string.

Create a subclass named Car that inherits from Vehicle and overrides the drive() method.

Print the output of calling drive() on both Vehicle and Car objects.

💡 Why This Matters

🌍 Real World

Open classes and methods let developers design flexible software that can grow and change without rewriting existing code.

💼 Career

Understanding open classes is important for Kotlin developers working on apps that require customization and extension, such as Android apps or libraries.

1

Create an open class with an open method

Create an open class called Vehicle with an open method named drive() that returns the string "Driving a vehicle".

Kotlin

# Your code here

Hint

Use the open keyword before the class and the method to allow them to be extended and overridden.

2

Create a subclass that overrides the method

Create a class called Car that inherits from Vehicle and overrides the drive() method to return the string "Driving a car".

Kotlin

open class Vehicle {
    open fun drive(): String {
        return "Driving a vehicle"
    }
}

# Your code here

Hint

Use class Car : Vehicle() to inherit and override fun drive() to change the method.

3

Create objects and call the methods

Create a variable vehicle as an instance of Vehicle and a variable car as an instance of Car. Call the drive() method on both and store the results in variables vehicleDrive and carDrive.

Kotlin

open class Vehicle {
    open fun drive(): String {
        return "Driving a vehicle"
    }
}

class Car : Vehicle() {
    override fun drive(): String {
        return "Driving a car"
    }
}

# Your code here

Hint

Create instances using val variableName = ClassName() and call methods with variable.method().

4

Print the results

Print the variables vehicleDrive and carDrive each on a new line.

Kotlin

open class Vehicle {
    open fun drive(): String {
        return "Driving a vehicle"
    }
}

class Car : Vehicle() {
    override fun drive(): String {
        return "Driving a car"
    }
}

val vehicle = Vehicle()
val car = Car()
val vehicleDrive = vehicle.drive()
val carDrive = car.drive()

# Your code here

Hint

Use println() to show the text on the screen.

Practice

1. In Kotlin, what does the open keyword do when applied to a class or method?

easy

A. Prevents the class or method from being used

B. Makes the class or method private

C. Allows the class or method to be inherited or overridden

D. Automatically overrides the method in subclasses

Solution

1. Step 1: Understand Kotlin class and method defaults

   By default, Kotlin classes and methods are final, meaning they cannot be inherited or overridden.

2. Step 2: Role of the open keyword

   The open keyword explicitly allows a class to be inherited or a method to be overridden in subclasses.

3. Final Answer:

   Allows the class or method to be inherited or overridden -> Option C

4. Quick Check:

   open means inheritance allowed [OK]

Hint: Remember: open means 'can be changed' [OK]

Common Mistakes:

• Thinking classes are open by default
• Confusing open with override
• Assuming open makes methods private

2. Which of the following is the correct way to declare an open method named display in Kotlin?

easy

A. open fun display() { }

B. fun display() open { }

C. fun open display() { }

D. open display fun() { }

Solution

1. Step 1: Recall Kotlin method declaration syntax

   In Kotlin, the keyword fun starts the function declaration, followed by the function name and parentheses.

2. Step 2: Position of open keyword

   The open keyword must come before fun to mark the method as open for overriding.

3. Final Answer:

   open fun display() { } -> Option A

4. Quick Check:

   open fun is correct order [OK]

Hint: Put open before fun for methods [OK]

Common Mistakes:

• Placing open after fun
• Putting open after method name
• Using open without fun keyword

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

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

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

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

medium

A. Some sound

B. Bark

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand class inheritance and method overriding

   The class Dog inherits from Animal and overrides the sound() method to return "Bark".

2. Step 2: Polymorphism in action

   Even though pet is typed as Animal, it holds a Dog instance, so the overridden sound() method in Dog is called.

3. Final Answer:

   Bark -> Option B

4. Quick Check:

   Overridden method runs on subclass instance [OK]

Hint: Overridden methods run on actual object type [OK]

Common Mistakes:

• Expecting base class method output
• Confusing compile-time and run-time types
• Forgetting to mark methods open

4. Identify the error in the following Kotlin code:

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

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

medium

A. Method start() in Car must be marked override

B. No error, code is correct

C. Car class cannot inherit from Vehicle

D. Vehicle class must be open to allow inheritance

Solution

1. Step 1: Check if Vehicle class allows inheritance

   By default, Kotlin classes are final. Since Vehicle is not marked open, it cannot be inherited.

2. Step 2: Analyze the inheritance attempt

   Car tries to inherit from Vehicle, which causes a compilation error because Vehicle is not open.

3. Final Answer:

   Vehicle class must be open to allow inheritance -> Option D

4. Quick Check:

   Classes must be open to inherit [OK]

Hint: Mark classes open to inherit them [OK]

Common Mistakes:

• Forgetting to mark base class open
• Assuming methods override without keyword
• Ignoring Kotlin's default final classes

5. You want to create a Kotlin class Shape with an open method area() that returns 0.0 by default. Then create a subclass Circle that overrides area() to return the circle's area using its radius. Which code correctly implements this?

hard

A. open class Shape {
open fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
override fun area() = 3.14 * radius * radius
}

B. class Shape {
fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
fun area() = 3.14 * radius * radius
}

C. open class Shape {
fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
override fun area() = 3.14 * radius * radius
}

D. class Shape {
open fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
override fun area() = 3.14 * radius * radius
}

Solution

1. Step 1: Mark base class and method open

   To allow inheritance and overriding, Shape must be open and its area() method must also be open.

2. Step 2: Override method correctly in subclass

   Circle inherits from Shape and overrides area() using override keyword, calculating area using radius.

3. Final Answer:

   Option A code correctly implements open class and method with override -> Option A

4. Quick Check:

   Open base and override subclass method [OK]

Hint: Open base class and method, override in subclass [OK]

Common Mistakes:

• Not marking class or method open
• Missing override keyword in subclass
• Trying to override non-open method