5. Classes and Objects

Class

Defined with the keyword class.

class Car {
    var speed: Int = 300
    var mileage: Int = 50
    var isHatchback: Boolean = false
    
    fun printDetails(){
        println("Your car has a max speed of $speed and mileage of $mileage. It is a ${if (isHatchback) "hatchback" else "sedan"}")
    }
}

fun main() {
    val jetta: Car = Car()
    jetta.printDetails()
    // OUTPUT: Your car has a max speed of 300 and mileage of 50. It is a sedan

    println(jetta.speed) // Prints 300 as getter function is created for the class under the hood by Kotlin.
    jetta.speed = 250 // Modifies speed as setter function is also created under the hood.
    println(jetta.speed) // Prints 250
}

Constructor

class Car(speed: Int = 300, mileage: Int = 50, isHatchBack: Boolean = false) {
    var speed: Int = speed
    var mileage: Int = mileage
    var isHatchback: Boolean = isHatchBack
    // ...

    // Init blocks can be used when more code needs to be executed 
    // with the constructor. 
    init{
        println("Obj initialized with values $speed $mileage $isHatchback ")
        // Prints: "Obj initialized with values 300 60 true" for Car(mileage = 60, isHatchBack = true) 
    }

}

fun main() {
    val polo: Car = Car(mileage = 60, isHatchBack = true) // Uses the default value 300 for speed
    polo.printDetails()
}

Secondary constructors

Used to perform constructor overloading.

class Car(speed: Int = 300, mileage: Int = 50, isHatchBack: Boolean = false) {
    // ...

    constructor(isRollsRoyce: Boolean) : this() {
        if(isRollsRoyce){
		    // Assigning special values for Rolls Royce
    		speed = 500
        	mileage = 50
	        isHatchback = false
		}
    }
    
    // ...
}

fun main() {
    // Uses the secondary constructor to define its values
    val royce: Car = Car(isRollsRoyce = true)
}

Getters and setters

Can be used to execute code while getting/setting individual values.

class Car(speed: Int = 300, mileage: Int = 50, isHatchBack: Boolean = false) {
    var mileage: Int = mileage
    var speed: Int = speed
    	get() = field //field is a keyword that references to speed in this case
    	set(value){
            if(value >= 500){
                mileage = 5
            }
            field = value
        }
    var isHatchback: Boolean = isHatchBack
    // ...
}

fun main() {
    val royce: Car = Car(isRollsRoyce = true)
    royce.speed = 500 // Calls the explicitly defined setter and sets speed to 500 as well as mileage to 5
    println(royce.mileage) // Prints 5
}

Visibility modifiers

• public = visible outside the class. Everything is public by default.
• internal means it will only be visible within the module. A module is a set of Kotlin files compiled together, for example, a library or application.
• private means it will only be visible in that class (or source file if you are working with functions).
• protected is the same as private, but it will also be visible to any subclasses.