Kotlin: Interface

An interface is like an abstract class, the difference is that an interface cannot store state.

Interface Declaration

interface Cats{
   fun doMeow()
}

How to use Interface

interface Cat {
    val sound: Sound
    fun doMeow() = sound.meow()
}

class Siamese: Cat{
    override val sound: Sound = Sound()
}
class Persian: Cat{
    override val sound: Sound = Sound()
}

class Sound{
    fun meow(){
        print("Meow meow")
    }
}

Note:

• An interface can’t have a constructor and cannot inherit from a class.

Multiple Interfaces in a Class

interface Cat {
    val sound: Sound
    var color: String
    var weight: Double
    fun doMeow() = sound.meow()
}
interface Animal{
    fun kind() : String = "Undefined"
}
//Adding new interface in a class using ,
class Siamese: Cat, Animal{
    val kind = "Carnivore"

    override var color: String = "Black"
    override var weight: Double = 3.5
    override val sound: Sound = Sound()
    override fun kind(): String {
        return kind
    }
}
class Persian: Cat{
    override var color: String = "White"
    override var weight: Double = 4.0
    override val sound: Sound = Sound()
}

open class Sound{
    fun meow(){
        println("Meow meow")
    }
}

Interfaces can implement another interface

interface Cat:Animal {
    val sound: Sound
    var color: String
    var weight: Double
    fun doMeow() = sound.meow()
}
interface Animal{
    fun kind() : String = "Undefined"
}
class Persian: Cat{
    override var color: String = "White"
    override var weight: Double = 4.0
    override val sound: Sound = Sound()
    //this will be redundant if you dont assign new value
    override fun kind(): String {
        return super.kind()
    }
}

I’m still learning, please leave a comment if there’s something I need to improve/learn/elaborate. Arigato!