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.

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