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

Abstract classes and methods

📖 Scenario: You are creating a simple program to represent different types of vehicles. Each vehicle has a way to describe itself, but the exact description depends on the type of vehicle.

🎯 Goal: Build an abstract class called Vehicle with an abstract method describe(). Then create two classes Car and Bike that inherit from Vehicle and provide their own description. Finally, print the descriptions of both vehicles.

📋 What You'll Learn

Create an abstract class called Vehicle

Add an abstract method describe() that returns a String

Create a class Car that inherits from Vehicle and implements describe()

Create a class Bike that inherits from Vehicle and implements describe()

Create instances of Car and Bike

Print the result of calling describe() on both instances

💡 Why This Matters

🌍 Real World

Abstract classes help organize code when you have different types of things that share some common behavior but also have their own specific details.

💼 Career

Understanding abstract classes and methods is important for designing clean and reusable code in many software development jobs.

1

Create the abstract class

Create an abstract class called Vehicle with an abstract method describe() that returns a String.

Kotlin

abstract class Vehicle {
    // Your code here
}

Hint

Use the abstract keyword before the class and the method. The method should have no body.

2

Create the Car class

Create a class called Car that inherits from Vehicle and implements the describe() method to return the string "This is a car.".

Kotlin

abstract class Vehicle {
    abstract fun describe(): String
}

// Your code here

Hint

Use class Car : Vehicle() to inherit and override fun describe() to provide the method body.

3

Create the Bike class

Create a class called Bike that inherits from Vehicle and implements the describe() method to return the string "This is a bike.".

Kotlin

abstract class Vehicle {
    abstract fun describe(): String
}

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

// Your code here

Hint

Similar to Car, inherit from Vehicle and override describe().

4

Create instances and print descriptions

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

Kotlin

abstract class Vehicle {
    abstract fun describe(): String
}

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

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

// Your code here

Hint

Create the instances with val car = Car() and val bike = Bike(). Use println() to print their descriptions.

Practice

1. What is true about an abstract class in Kotlin?

easy

A. It cannot be instantiated directly and may contain abstract methods.

B. It can be instantiated like any other class.

C. It must not contain any methods with a body.

D. It is the same as an interface.

Solution

1. Step 1: Understand abstract class instantiation rules

   Abstract classes cannot create objects directly because they are incomplete by design.

2. Step 2: Check method types in abstract classes

   Abstract classes can have both abstract methods (without body) and concrete methods (with body).

3. Final Answer:

   It cannot be instantiated directly and may contain abstract methods. -> Option A

4. Quick Check:

   Abstract class = no direct objects + can have abstract methods [OK]

Hint: Abstract classes can't create objects directly [OK]

Common Mistakes:

• Thinking abstract classes can be instantiated
• Believing abstract classes can't have method bodies
• Confusing abstract classes with interfaces

2. Which of the following is the correct way to declare an abstract method in a Kotlin abstract class?

easy

A. abstract fun calculate(): Int

B. fun abstract calculate(): Int

C. fun calculate() abstract: Int

D. abstract calculate fun(): Int

Solution

1. Step 1: Recall Kotlin syntax for abstract methods

   In Kotlin, abstract methods are declared with the abstract keyword before fun, followed by the method signature without a body.

2. Step 2: Check each option's syntax

   Only abstract fun calculate(): Int matches the correct syntax: abstract fun calculate(): Int. Others have incorrect keyword order or punctuation.

3. Final Answer:

   abstract fun calculate(): Int -> Option A

4. Quick Check:

   abstract fun method() = correct syntax [OK]

Hint: abstract keyword comes before fun in method declaration [OK]

Common Mistakes:

• Placing abstract after fun
• Adding a method body to abstract methods
• Incorrect keyword order

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

abstract class Animal {
    abstract fun sound(): String
}

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

fun main() {
    val dog = Dog()
    println(dog.sound())
}

medium

A. Animal

B. sound

C. Bark

D. Compilation error

Solution

1. Step 1: Understand class inheritance and method override

   The abstract class Animal has an abstract method sound(). The subclass Dog overrides it to return "Bark".

2. Step 2: Analyze main function output

   Creating a Dog object and calling sound() prints "Bark".

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Overridden method returns "Bark" [OK]

Hint: Overridden abstract method returns subclass value [OK]

Common Mistakes:

• Expecting abstract class method to run
• Confusing method name with output
• Thinking abstract class can be instantiated

4. Identify the error in this Kotlin code:

abstract class Vehicle {
    abstract fun drive()
}

class Car : Vehicle() {
    fun drive() {
        println("Driving")
    }
}

medium

A. drive method in Vehicle should have a body

B. Vehicle class cannot be abstract

C. Car class cannot inherit from Vehicle

D. Missing override keyword in Car's drive method

Solution

1. Step 1: Check method overriding rules in Kotlin

   When a subclass implements an abstract method, it must use the override keyword.

2. Step 2: Identify missing override keyword

   The Car class defines drive() without override, causing a compilation error.

3. Final Answer:

   Missing override keyword in Car's drive method -> Option D

4. Quick Check:

   Override keyword required for abstract method implementation [OK]

Hint: Use override keyword when implementing abstract methods [OK]

Common Mistakes:

• Omitting override keyword
• Adding body to abstract method in base class
• Misunderstanding inheritance rules

5. Given this abstract class and subclass, what will be the output?

abstract class Shape {
    abstract fun area(): Double
    fun description() = "Shape area is ${area()}"
}

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

fun main() {
    val c = Circle(2.0)
    println(c.description())
}

hard

A. Shape area is 6.28

B. Shape area is 12.56

C. Shape area is 4.0

D. Compilation error due to abstract method

Solution

1. Step 1: Calculate area in Circle subclass

   Area = π * radius² = 3.14 * 2.0 * 2.0 = 12.56

2. Step 2: Call description method from Shape

   Description returns string with area value: "Shape area is 12.56".

3. Final Answer:

   Shape area is 12.56 -> Option B

4. Quick Check:

   Abstract method overridden, description uses area() [OK]

Hint: description calls overridden area() method [OK]

Common Mistakes:

• Forgetting to override abstract method
• Miscomputing area formula
• Expecting abstract class method to run alone