Abstract classes and methods in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an abstract class named Vehicle.

Kotlin

abstract class [1] {
    abstract fun start()
}

Drag options to blanks, or click blank then click option'

AVehicle

BCar

CDrive

DStart

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name instead of a class name.

Forgetting to use the abstract keyword.

The keyword abstract is used to declare an abstract class. Here, Vehicle is the class name.

2fill in blank

medium

Complete the code to declare an abstract method named drive inside an abstract class.

Kotlin

abstract class Vehicle {
    abstract fun [1]()
}

Drag options to blanks, or click blank then click option'

Arun

Bdrive

Cstop

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a method body to an abstract method.

Using a method name unrelated to the class.

The abstract method drive is declared without a body inside the abstract class.

3fill in blank

hard

Fix the error in the subclass that inherits from an abstract class and implements the abstract method.

Kotlin

abstract class Vehicle {
    abstract fun drive()
}

class Car : Vehicle() {
    override fun [1]() {
        println("Car is driving")
    }
}

Drag options to blanks, or click blank then click option'

Amove

Brun

Cstart

Ddrive

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than the abstract method.

Forgetting the override keyword.

The subclass Car must override the abstract method drive from the superclass Vehicle.

4fill in blank

hard

Fill both blanks to create an abstract class with an abstract method and a concrete method.

Kotlin

abstract class [1] {
    abstract fun [2]()
    fun stop() {
        println("Stopping")
    }
}

Drag options to blanks, or click blank then click option'

AMachine

Bstart

Cdrive

DVehicle

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete method name as abstract.

Naming the class with a verb instead of a noun.

The abstract class is named Machine. It has an abstract method start and a concrete method stop.

5fill in blank

hard

Fill all three blanks to implement an abstract class and override its abstract method in a subclass.

Kotlin

abstract class [1] {
    abstract fun [2]()
}

class [3] : [1]() {
    override fun [2]() {
        println("Running")
    }
}

Drag options to blanks, or click blank then click option'

ARunner

Brun

CAthlete

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for class and subclass.

Not overriding the abstract method correctly.

The abstract class is Runner with an abstract method run. The subclass Athlete overrides run.

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