Abstract classes and methods in Kotlin - Time & Space Complexity

Time Complexity: Abstract classes and methods

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run code using abstract classes and methods changes as the input grows.

Specifically, we ask: how does using abstract methods affect the speed of running a program?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

abstract class Shape {
    abstract fun area(): Double
}

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

fun totalArea(shapes: List<Shape>): Double {
    var sum = 0.0
    for (shape in shapes) {
        sum += shape.area()
    }
    return sum
}

This code defines an abstract class with an abstract method, then calculates the total area of many shapes by calling the method on each.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the list of shapes and calling the area() method on each.
• How many times: Once for each shape in the list (n times if the list has n shapes).

How Execution Grows With Input

As the number of shapes increases, the program calls the area() method more times, one per shape.

Input Size (n)	Approx. Operations
10	10 calls to area()
100	100 calls to area()
1000	1000 calls to area()

Pattern observation: The number of operations grows directly with the number of shapes.

Final Time Complexity

Time Complexity: O(n)

This means the time to compute total area grows in a straight line with the number of shapes.

Common Mistake

[X] Wrong: "Using abstract methods makes the program slower by a lot because of extra overhead."

[OK] Correct: Calling abstract methods adds only a tiny fixed cost per call, so the overall time still grows mainly with the number of shapes, not the method type.

Interview Connect

Understanding how abstract methods affect time helps you explain design choices clearly and shows you know how code structure relates to performance.

Self-Check

What if we added a nested loop inside the area() method that runs proportional to the size of some input? How would the time complexity change?

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