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Cover image for #3 SOLID - Liskov Substitution Principle | Swift | iOS Development
Bibin Jaimon
Bibin Jaimon

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#3 SOLID - Liskov Substitution Principle | Swift | iOS Development

LSP states that objects of a superclass should be replaceable with objects of its subclass without affecting the correctness of the program - Barbara Liskov (1987)

In the below example,

  • We have a class hierarchy representing different shapes: a superclass Shape and two subclasses, Rectangle and Circle.
  • The Shape class acts as a superclass defining a common interface with the area() method.
  • Both Rectangle and Circle subclasses inherit from Shape and provide their own implementation of the area() method.
class Shape {
    func area() -> Double {
        fatalError("Subclasses must override area() method.")

class Rectangle: Shape {
    var width: Double
    var height: Double

    init(width: Double, height: Double) {
        self.width = width
        self.height = height

    override func area() -> Double {
        return width * height

class Circle: Shape {
    var radius: Double

    init(radius: Double) {
        self.radius = radius

    override func area() -> Double {
        return Double.pi * radius * radius
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  • In the printArea() function, we accept an instance of Shape as a parameter.
  • The area() method will be dynamically dispatched based on the actual type of the object at runtime, and the correct implementation will be executed.
  • This example demonstrates how the Liskov Substitution Principle allows us to treat objects of different subclasses as interchangeable, as long as they conform to the common superclass interface.
func printArea(shape: Shape) {
    let area = shape.area()
    print("Area: \(area)")

let rectangle = Rectangle(width: 4, height: 5)
let circle = Circle(radius: 3)

printArea(shape: rectangle) // Output: Area: 20.0
printArea(shape: circle)    // Output: Area: 28.274333882308138
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