Abstract classes and methods in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract class method call

What is the output of this Kotlin code?

Kotlin

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

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

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

A Bark

B Sound

C Compilation error

D Runtime error

💡 Hint

Remember that abstract methods must be overridden in subclasses.

The Dog class overrides the abstract sound() method and returns "Bark". So, calling dog.sound() prints "Bark".

🧠 Conceptual

intermediate

Abstract class instantiation

Which statement about abstract classes in Kotlin is true?

A All methods in an abstract class must be abstract.

B Abstract classes cannot have constructors.

C Abstract classes can have both abstract and non-abstract methods.

D You can create an instance of an abstract class directly.

💡 Hint

Think about what abstract classes are used for and their flexibility.

Abstract classes can have both abstract methods (without body) and concrete methods (with body). You cannot instantiate abstract classes directly, but they can have constructors.

🔧 Debug

advanced

Identify the error in abstract method override

What error will this Kotlin code produce?

Kotlin

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

class Car : Vehicle() {
    fun move(): String {
        return "Driving"
    }
}

fun main() {
    val car = Car()
    println(car.move())
}

A Compilation error: 'move' in 'Car' hides abstract member in 'Vehicle' and must be marked override

B Runtime error: Abstract method not implemented

C No error, output: Driving

D Compilation error: Cannot instantiate abstract class

💡 Hint

Check if the method in subclass properly overrides the abstract method.

The move() method in Car is missing the override keyword. Kotlin requires explicit override for abstract methods.

📝 Syntax

advanced

Correct syntax for abstract class with constructor

Which option shows the correct way to declare an abstract class with a primary constructor in Kotlin?

A

abstract class Shape {
    constructor(name: String)
    abstract fun area(): Double
}

B

abstract class Shape(val name: String) {
    abstract fun area(): Double
}

C

abstract class Shape() {
    val name: String
    abstract fun area(): Double
}

D

abstract class Shape {
    val name: String
    abstract fun area(): Double
    constructor(name: String) { this.name = name }
}

💡 Hint

Remember how primary constructors are declared in Kotlin classes.

Option B correctly declares an abstract class with a primary constructor and an abstract method. Other options have syntax errors or missing initialization.

🚀 Application

expert

Determine the output of complex abstract class hierarchy

What is the output of this Kotlin program?

Kotlin

abstract class A {
    abstract fun f(): String
    fun g() = "G from A"
}

abstract class B : A() {
    override fun f() = "F from B"
    abstract fun h(): String
}

class C : B() {
    override fun h() = "H from C"
}

fun main() {
    val c = C()
    println(c.f())
    println(c.g())
    println(c.h())
}

A Runtime error: Abstract method h() not implemented

B

F from B
G from B
H from C

C Compilation error: Class C must override abstract method g()

D

F from B
G from A
H from C

💡 Hint

Trace the inheritance and method overrides carefully.

Class C inherits f() from B, g() from A, and overrides h(). So the output matches option D.