Android Programming – Part 9 Kotlin Fundamentals Ⅸ: OOP

Hi all, it’s been three days since I last posted here… Well, to be honest, I lost some motivation to keep up with my coding series here since I’m more inclined to do my own personal projects. BUT, I’ll start posting things here too. At least, I have to finish up Kotlin’s basic learning processes to start my own asap.

Anyways, let’s go!!

Kotlin’s OOP:

This is our first Kotlin’s OOP programming. As I assume that you can understand the basics of Java or other programming languages, I skip explaining this.

classSample1.kt

class Car() {
    val color : String = "blue"
}

fun main() {
    val car = Car()
    println(car.color)
}

$java -jar classSample1.jar
blue

And I added a function.

classSample1.kt

class Car() {
    val color : String = "blue"
    fun drive() {
        println("VMMMMMMMMMMMMMM!!!")
    }
}

fun main() {
    val car = Car()
    println(car.color)
    println(car.drive())
}

$java -jar classSample1.jar
blue
VMMMMMMMMMMMMMM!!!
kotlin.Unit

And now the Car class receives arguments.

classSample1.kt

class Car(color : String = "blue", model : String = "XMD") {
    var color : String = color
    var model : String = model
}

fun main() {
    val car = Car(color = "RED", model = "TOYOTA")
    println(car.color)
    println(car.model)
}

$java -jar classSample1.jar
RED
TOYOTA

Init block:

Okay, so from here, we’ll look a bit deeper into Kotlin’s world. Inti block is a block of code that is executed when an object is created, similar to Java’s constructor. Here’s an example.

classSample1.kt

class Car(var color : String, var model : String) {
    init {
        color = "Yellow"
        model = "Lexus"
    }
}

fun main() {
    val car = Car(color = "RED", model = "TOYOTA")
    println(car.color)
    println(car.model)
}

$java -jar classSample1.jar
Yellow
Lexus

Inheritance:

classSample1.kt

Inheritance is one of the core concepts of OOP, isn’t it? As you may be already aware of this, what is a powerful feature of inheritance that can be seen in the below code is the fact that Truck’s object can access to Car’s functions, drive, and speedMeter.

open class Car(var color : String, var model : String) {
    init {
        color = "Yellow"
        model = "Lexus"
    }

    fun drive() {
        println("VMMMMMMMMMMMMM")
    }

    fun speedMeter(minSpeed : Int, maxSpeed : Int) {
        println("drive: " + minSpeed + " " + maxSpeed)
    }
}

class Truck(color : String, model : String) : Car (color, model) {

    fun lightUp() {
        println("Pika Pika")
    }
}

fun main() {
    val car = Car(color = "RED", model = "TOYOTA")
    println(car.color)
    println(car.model)
    val truck = Truck(color = "White", model = "Suzuki")
    println(truck.lightUp())
    println(truck.drive())
    println(truck.speedMeter(20, 30))
}

$java -jar inheritanceSample1.jar
Yellow
Lexus
Pika Pika
kotlin.Unit
VMMMMMMMMMMMMM
kotlin.Unit
drive: 20 30
kotlin.Unit

Override:

Okay, this is another OOP feature that doesn’t need any explanation. But there, as you can see Truck’s drive function overrides the original one.

classSample1.kt

open class Car(var color : String, var model : String) {
    init {
        color = "Yellow"
        model = "Lexus"
    }

    open fun drive() {
        println("VMMMMMMMMMMMMM")
    }

    fun speedMeter(minSpeed : Int, maxSpeed : Int) {
        println("drive: " + minSpeed + " " + maxSpeed)
    }
}

class Truck(color : String, model : String) : Car (color, model) {

    fun lightUp() {
        println("Pika Pika")
    }

    override fun drive() {
        println("VMMMMMMMMMMMMM ... like a truck")
    }
}

fun main() {
    val car = Car(color = "RED", model = "TOYOTA")
    println(car.color)
    println(car.model)
    val truck = Truck(color = "White", model = "Suzuki")
    println(truck.lightUp())
    println(truck.drive())
    println(truck.speedMeter(20, 30))
}

$ java -jar inheritanceSample1.jar
Yellow
Lexus
Pika Pika
kotlin.Unit
VMMMMMMMMMMMMM ... like a truck
kotlin.Unit
drive: 20 30
kotlin.Unit

Extention function:

This is a Kotlin-only feature that probably can’t be seen in Java, I guess…

extentionFunSample.kt

fun String.append(toAppend: String): String{
    return this.plus(toAppend)
}

fun main() {
    println("Hello, Jenny".append("Miss"))

}

$ java -jar extentionFunSample.jar
Hello, JennyMiss

extentionFunSample2.kt (shorter version)

And you can make it even shorter.

fun String.append(toAppend: String): String = this.plus(toAppend)

fun main() {
    println("Hello, Jenny".append("Miss"))

}

$ java -jar extentionFunSample2.jar
Hello, JennyMiss

Data class

Data class is Kotlin’s equivalent of C’s structure, I guess. Early in the year, I posted about it in the C programming series, so if you have some extra time, please check it out too: Learn C Language – Part 4: Structure.

dataClassSample.kt

fun main() {
    var person = Person(
        name = "John",
        lastName = "Doe",
        age = 23
    )
    println(person)
}

data class Person(val name: String, val lastName: String, val age: Int)

$ java -jar dataClassSample.jar
Person(name=John, lastName=Doe, age=23)

And you can loop through multiple data objects.

dataClassSample2.kt

fun main() {
    var person = Person(
        name = "John",
        lastName = "Doe",
        age = 23
    )

    val person2 = Person(name = "Katy", lastName = "Holms", age = 33)

    val personList = listOf(person, person2)

    for (i in personList) {
        println(i)
    }

    
}

data class Person(val name: String, val lastName: String, val age: Int)

$ java -jar dataClassSample2.jar
Person(name=John, lastName=Doe, age=23)
Person(name=Katy, lastName=Holms, age=33)