Revising Basic Design Patterns: Composition in Kotlin

Hello, welcome back! Today, we will decode the fundamentals of Revising Basic Design Patterns — Composition! A vital component of software design patterns, composition aids us in creating complex classes using simpler ones. Our journey today includes understanding the concept of composition, its value in software development, and how to practically implement it in Kotlin.

To kick-start our exploration, let's understand Composition. In object-oriented programming (OOP), composition allows a class to include other classes, paving the way for creating complex systems out of simpler components. For instance, when building a car, we bring together independent pieces like the Engine, Wheels, and Seats — a perfect reflection of composition in everyday life. Note that in composition, if the parent object (the car) is destroyed, the child objects (the components) also cease to exist.

Now, let's translate the theory into a Kotlin code application. Transforming the previously mentioned car example, a Car class in Kotlin is created by making objects of the Engine, Wheels, and Seats classes. The Car class owns these child objects; their existence depends on the Car.

class Engine {
    fun start() {
        println("Engine starts")  // Engine start message
    }

    fun stop() {
        println("Engine stops")   // Engine stop message
    }
}

class Wheels {
    fun rotate() {
        println("Wheels rotate")  // Wheel rotation message
    }
}

class Seats {
    fun adjust(position: String) {
        println("Seats adjusted to position $position") // Seat adjustment message
    }
}

class Car(private val engine: Engine, private val wheels: Wheels, private val seats: Seats) {

    fun start() {
        engine.start() // Call to start engine
        seats.adjust("upright") // Adjust seat position
        wheels.rotate() // Get wheels rolling
    }
}

fun main() {
    val engine = Engine()
    val wheels = Wheels()
    val seats = Seats()
    val myCar = Car(engine, wheels, seats)
    myCar.start()  // Begin car functions
}
// Prints:
// Engine starts
// Seats adjusted to position upright
// Wheels rotate

In the above code, the Car class encapsulates Engine, Wheels, and Seats objects, which are independent but part of the Car class, forming a composition pattern.

In the composition model, components like Engine, Wheels, and Seats can exist independently. These classes are not subclasses of Car, but are simply used by the Car to achieve its functionality. This is an important feature of composition because it allows for the reuse of these components in other contexts. For instance, the Engine class could be used in other vehicles like a Truck or Boat, without needing to inherit from a specific Car class.