Kotlin Class and Object Examples

In this tutorial you will learn about classes and objects via simple step by step examples.

Example 1: DataClass

Let us look at a simple DataClass example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• DataClass.kt

1. In your editor or IDE, create a file known as DataClass.kt.
2. Then add the following code:

(a). DataClass.kt

package Objects.DataClass

/* This sample demonstrates various features that you get from using a data class */

public data class Superhero(val firstName : String, val lastName : String)

fun main(args : Array<String>) {
    var superman = Superhero("clark", "kent")

    //Automatic toString generation
    println("Superman is  ${superman.toString()}")

    //Automatic hashCode
    println("Superman hashcode is ${superman.hashCode()}")

    var supermanAlias = Superhero("clark", "kent")

    //Auto generated equals
    var areTheyEqual = superman.equals(supermanAlias)
    println("Is superman equals supermanAlias? $areTheyEqual")

    //component methods are generated
    val isFirstName = superman.firstName.equals(superman.component1())
    println("is .firstName equal to .component1? $isFirstName ")

    var isLastName = superman.lastName.equals(superman.component2())
    println("is .lastName equals to .component2? $isLastName")

    //multi declarations
    val hero = { x : Superhero -> x }
    val(firstName, lastName ) = hero(superman)

    println("Our superhero name is $firstName $lastName")

    //another sample of multi declarations
    fun supercharge(x : Superhero) : Superhero {
        var m = Superhero(x.firstName.toUpperCase(), x.lastName.toUpperCase())
        return m
    }

    val(firstName2, lastName2) = supercharge(superman)
    println("Our supercharged superhero name is $firstName2 $lastName2")
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 2: Constructors

Let us look at a simple Constructors example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• Constructors.kt

1. In your editor or IDE, create a file known as Constructors.kt.
2. Then add the following code:

(a). Constructors.kt

package Objects.Constructors

class Primary(initialName : String, age : Int = 30) {
    var firstName = initialName
    val age = age

    init {
        //this is an anonymous constructor
        //there is no other type of constructor
        firstName += ".jr"
    }

    public fun sayName() {
        println("My name is $firstName and I am $age years old.")
    }
}

fun main(args : Array<String>) {
    var p = Primary("John Adams", 56)
    p.sayName()

    var n = Primary("Bon Jovi")
    n.sayName()
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 3: Object Properties

Let us look at a simple Object Properties example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• Properties.kt

1. In your editor or IDE, create a file known as Properties.kt.
2. Then add the following code:

(a). Properties.kt

package Objects.Properties

fun main(args : Array<String>) {
    //you cannot change johnClark name. It's immutable
    var johnC = Person("John", "Clark")
    println("The name is ${johnC.firstName} ${johnC.lastName}")

    var markC = Soldier("Mark", "Chavez")
    println("The name is ${markC.firstName} ${markC.lastName}")

    var vanD = UniversalSoldier("Van", "Damme")
    println("The name is ${vanD.firstName} ${vanD.lastName}")

    var slyvesterS = UltimateSoldier("Slyvester", "Stallone")
    println("The name is ${slyvesterS.firstName} ${slyvesterS.lastName}")
}

//This will make two immutable properties
class Person(val firstName : String, val lastName : String)

//This is another way to declare the immutable properties.
class Soldier(firstName : String, lastName : String) {
    public val firstName : String = firstName
    public val lastName : String = lastName
}

//Property with backing field
//The compiler only generates a backing field if it is used by the accessors.
class UniversalSoldier(firstName : String, lastName : String) {
    public var firstName : String = firstName
        get() {
            return "Universal " + field
        }
        set(value) {
            field = value
        }

    public var lastName : String = lastName
}

//property with old java style
class UltimateSoldier(firstName : String, lastName : String) {
    private var _firstName = ""
    public var firstName : String
        get() {
            return _firstName
        }
        set(value) {
            _firstName = value
        }

    private var _lastName = ""
    public var lastName : String
        get() {
            return _lastName
        }
        set(value) {
            _lastName = value
        }

    init {
        //constructor - we cannot initialize directly to the properties because this way there is no backing field
        this.firstName = firstName
        this.lastName = lastName
    }
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 4: ObjectClass

Let us look at a simple ObjectClass example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• ObjectClass.kt

1. In your editor or IDE, create a file known as ObjectClass.kt.
2. Then add the following code:

(a). ObjectClass.kt

package Objects.ObjectClass

fun main(args : Array<String>) {

    val espana = Matador("Emilio")

    Matador.show(espana)

    //you can also assign class object to a variable and use it later
    val m = Matador
    println("Typeof " + m)
    m.show(espana)

    //how do you pass it to a parameter
}

class Matador(name : String) {
    private val name : String = name

    private fun myPrivateShow() {
        println("This is ${name} private show")
    }

    companion object {
        fun show(mt : Matador) {
            //function inside a class object can access private properties and function of the class
            println ("Expose the private secret of ${mt.name}")
            mt.myPrivateShow()
        }
    }
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 5: ExtensionProperties

Let us look at a simple ExtensionProperties example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• ExtensionProperties.kt

1. In your editor or IDE, create a file known as ExtensionProperties.kt.
2. Then add the following code:

(a). ExtensionProperties.kt

package Objects.ExtensionProperties

val Amazing.isEmpty : Boolean
    get() = this.name.length == 0

class Amazing(name : String) {
    var name : String = name
}

fun main(args : Array<String>) {
    val italian = Amazing("Roberto")

    println("${italian.name} is  empty : ${italian.isEmpty}")
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 6: InvokableObjects

Let us look at a simple InvokableObjects example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• Invokable.kt

1. In your editor or IDE, create a file known as Invokable.kt.
2. Then add the following code:

(a). Invokable.kt

package Objects.InvokableObjects

/* You can use any type that has an operator function named invoke() in a callee position:*/

interface Superhero {
    public operator fun invoke()
}

class Batman : Superhero {
    override operator fun invoke() {
        println ("Batman punches")
    }
}

class Superman {
}

// this also works even if it's just  an extension function
operator fun Superman.invoke() {
    println ("Superman flies")
}

fun invoke(x : Batman) {
    x()
}

fun invoke2(x : Superhero) {
    x()
}

fun invoke3(x : Superman) {
    x()
}

fun main(args : Array<String>) {
    var darkNight = Batman()
    var clark = Superman()

    invoke(darkNight)//direct type
    invoke2(darkNight)//via trait
    invoke3(clark)//via extension function
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 6: Inheritance

Let us look at a simple Inheritance example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• Inheritance.kt

1. In your editor or IDE, create a file known as Inheritance.kt.
2. Then add the following code:

(a). Inheritance.kt

package Objects.Inheritance

fun main(args : Array<String>) {
    //simple final class
    var final = FinalConstruct("Andrew Sullivan")
    println("His name is ${final.name}")

    //derived class with two traits
    var less = LessFlexible("Karl Rove")
    println("His name is ${less.name}")
    less.dance()
    less.sing()
}

class FinalConstruct(var name : String) {
}

open class FlexibleConstruct(var name : String) {

}

interface Singing {
    fun sing() {
        println("I can sing")
    }
}

interface Dancing {
    fun dance() {
        println("I can dance")
    }
}

//You can only have one supertype but multiple interfaces
class LessFlexible(name : String) : FlexibleConstruct(name), Singing, Dancing {

}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 7: How to copy a data class

Let us look at a simple example of how to copy a data class. The example is written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• CopyDataClass.kt

1. In your editor or IDE, create a file known as CopyDataClass.kt.
2. Then add the following code:

(a). CopyDataClass.kt

package Objects.CopyDataClass

data class Person(val firstName: String, val lastName: String){
    public fun print(){
        println("$firstName $lastName")
    }
}

fun main(args : Array<String>) {
    val presidentOne = Person("Bill", "Clinton")
    presidentOne.print()
    val presidentTwo = presidentOne.copy(firstName = "Hillary")
    presidentTwo.print()
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 8: Delegation with Interfaces

Let us look at a simple Delegation via interfaces example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• Delegation.kt

1. In your editor or IDE, create a file known as Delegation.kt.
2. Then add the following code:

(a). Delegation.kt

package Objects.Delegation

//This is a neat little feature being able to automatically assign an object to handle any interface

fun main(args : Array<String>) {
    var me = BruceWayne(Batman(), RichyRich())
    me.inWater()
    print("Is awesomely rich? " + me.isAwesomelyRich())
}

interface Superpower {
    fun inWater()
    fun onAir()
    fun onSoil()
}

interface Wealth {
    fun isAwesomelyRich() : Boolean
}

public class Batman() : Superpower {
    companion object {
        fun create() = Batman()
    }

    override fun inWater() {
        println("Ack, cannot swim")
    }

    override fun onAir() {
        println("Requires vehicle")
    }

    override fun onSoil() {
        println("Awesome")
    }

    fun isFun() : Boolean {
        return false
    }
}

public class RichyRich : Wealth {
    override fun isAwesomelyRich() : Boolean {
        return true
    }
}

public class BruceWayne(a : Batman, b : RichyRich) : Superpower by a, Wealth by b

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 9: Nested class

Let us look at a simple Nested class example written in Kotlin. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• Nested.kt

1. In your editor or IDE, create a file known as Nested.kt.
2. Then add the following code:

(a). Nested.kt

package Objects.Nested

fun main(args : Array<String>) {
    var btm = Batman()
    var rbn = Batman.Robin()

    btm.say()
    rbn.say()
}

public class Batman() {
    fun say() {
        println("This is Batman")
    }

    public class Robin {
        fun say() {
            println("This is Robin")
        }
    }
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Example 10: Object Expressions

Let us look at a simple Object Expressions example written in Kotlin. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• ObjectExpressions.kt

1. In your editor or IDE, create a file known as ObjectExpressions.kt.
2. Then add the following code:

(a). ObjectExpressions.kt

package Objects.Expressions

fun main(args : Array<String>) {
    var alone = object {
        val name : String = "Broken Glass House"
        val address : String = "55 N. State Street, Chicago"
    }

    println ("This is the address of the event : '${alone.name}' with address '${alone.address}'")

    var killer = object : Player("John Adams") {
        override fun fight() {
            println ("$name can shoot and run")
        }
    }

    killer.fight()
}

fun anythingGoes() : Any {
    return object {
        val message : String = "You can use this anytime"
    }
}

open class Player(name : String) {
    public val name : String = name

    public open fun fight() {
        println("$name can fight.")
    }
}

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links:

Kotlin Visibility Examples

In this tutorial you will learn about Visibility via simple step by step examples.

Let us look at a simple Visibility example written in Kotlin Programming Language. Follow the following steps:

Step 1: Create Project

1. Open your favorite Kotlin IDE.
2. In the menu go to File --> Create New Project.

Step 2: Add Dependencies

No dependencies are needed for this project.

Step 3: Write Code

Our code will comprise the following Kotlin files:

• Cookie.kt
• MultipleRef.kt
• ObserveAnimals.kt
• RecordAnimals.kt

Example 1: Private Member Example

1. In your editor or IDE, create a file known as Cookie.kt.
2. Then add the following code:

class Cookie(
  private var isReady: Boolean  // [1]
) {
  private fun crumble() =       // [2]
    println("crumble")

  public fun bite() =           // [3]
    println("bite")

  fun eat() {                   // [4]
    isReady = true              // [5]
    crumble()
    bite()
  }
}

fun main() {
  val x = Cookie(false)
  x.bite()
  // Can't access private members:
  // x.isReady
  // x.crumble()
  x.eat()
}
/* Output:
bite
crumble
bite
*/

Example 2: MultipleRef.kt

1. Next create another file known as MultipleRef.kt.
2. And add the following code:

// Visibility/MultipleRef.kt

class Counter(var start: Int) {
  fun increment() {
    start += 1
  }
  override fun toString() = start.toString()
}

class CounterHolder(counter: Counter) {
  private val ctr = counter
  override fun toString() =
    "CounterHolder: " + ctr
}

fun main() {
  val c = Counter(11)                 // [1]
  val ch = CounterHolder(c)           // [2]
  println(ch)
  c.increment()                       // [3]
  println(ch)
  val ch2 = CounterHolder(Counter(9)) // [4]
  println(ch2)
}
/* Output:
CounterHolder: 11
CounterHolder: 12
CounterHolder: 9
*/

Example 3: Animals Example

1. Next create another file known as RecordAnimals.kt.
2. And add the following code:

(d). RecordAnimals.kt

// Visibility/RecordAnimals.kt

private var index = 0                  // [1]

private class Animal(val name: String) // [2]

private fun recordAnimal(              // [3]
  animal: Animal
) {
  println("Animal #$index: ${animal.name}")
  index++
}

fun recordAnimals() {
  recordAnimal(Animal("Tiger"))
  recordAnimal(Animal("Antelope"))
}

fun recordAnimalsCount() {
  println("$index animals are here!")
}

ObserveAnimals.kt

1. Next create another file known as ObserveAnimals.kt.
2. And add the following code:

// Visibility/ObserveAnimals.kt

fun main() {
  // Can't access private members
  // declared in another file.
  // Class is private:
  // val rabbit = Animal("Rabbit")
  // Function is private:
  // recordAnimal(rabbit)
  // Property is private:
  // index++

  recordAnimals()
  recordAnimalsCount()
}
/* Output:
Animal #0: Tiger
Animal #1: Antelope
2 animals are here!
*/

Step 4: Run

Copy the code, build and run.

Reference

Here are the reference links: