Kotlin OOP Class and Objects

Class and object are basic building blocks in object-oriented programming languages. A class is written by a programmer in a defined structure to create an object (computer science) in an object oriented programming language. It defines a set of properties and methods that are common to all objects of one type.

For example, a class could be a car, which could have a color field, make field and model field. Another related class could be a truck, which would have similar fields, but not be exactly the same as a car. Both a car and a truck could be a kind of a third class which could be called a vehicle class. In this way, the programmer could create the parts of the program that are the same for both the car and the truck in programming the vehicle class, yet the programmer would be free to program how a car is different from a truck without duplicating all of the programming.

Kotlin class syntax

class ClassName {
    // property
    // member function
    ... .. ...
}

Example Kotlin Class

class Car {
    var color: String = "Silver" //Default value for colors cars is Silver
    var make: String = ""
    var model: String = ""

    get() = field

    set(value) {
        field = value
    }
}


class Truck{
    var color: String = "Silver" //Default value for colors trucks is Silver
    var make: String = ""
    var model: String = ""

    get() = field

    set(value) {
        field = value
    }
}

Objects from class in Kotlin

When you instantiate object of the Car class and initialize the color, make and model properties, it is passed to the setters parameter value and sets field to value.

Example of creating an object from class in Kotlin

fun main() {

  val newCar = Car()

  newCar.color = "Red"
  newCar.make = "BMW"
  newCar.model = "X5"

  println("Car color is: " + newCar.color)
  println("Car make is: " + newCar.make)
  println("Car model is: " + newCar.model)
}

Another example of creating an object from class in Kotlin

fun main() {

  val newTruck = Truck()

  newTruck.color = "Blue"
  newTruck.make = "Chevy"
  newTruck.model = "Silverado"

  println("Truck color is: " + newTruck.color)
  println("Truck make is: " + newTruck.make)
  println("Truck model is: " + newTruck.model)
}

Complete Example Code

class Car {
    var color: String = "Silver" //Default value for colors cars is Silver
    var make: String = ""
    var model: String = ""

    get() = field

    set(value) {
        field = value
    }
}


class Truck{
    var color: String = "Silver" //Default value for colors trucks is Silver
    var make: String = ""
    var model: String = ""

    get() = field

    set(value) {
        field = value
    }
}

fun main() {

  val newCar = Car()

  newCar.color = "Red"
  newCar.make = "BMW"
  newCar.model = "X5"

  println("Car color is: " + newCar.color)
  println("Car make is: " + newCar.make)
  println("Car model is: " + newCar.model)

  val newTruck = Truck()

  newTruck.color = "Red"
  newTruck.make = "BMW"
  newTruck.model = "X5"

  println("Truck color is: " + newTruck.color)
  println("Truck make is: " + newTruck.make)
  println("Truck model is: " + newTruck.model)
}

Output

Car color is: Red
Car make is: BMW
Car model is: X5
Truck color is: Blue
Truck make is: Chevy
Truck model is: Silverado

Conclusion

In this page (written and validated by A. Gawali) you learned about Kotlin OOP Class and Objects . What's Next? If you are interested in completing Kotlin tutorial, your next topic will be learning about: Kotlin Class Constructor.

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