OOP.kt

package main
abstract class AB
interface INTF
open class X
data class Y:X()
sealed class Z
class A{
    class B
    inner class C
    object D{//can implement interfaces
        fun hello()=println("hello")
    }
    companion object E{//static behavior
         fun hello()=println("hello")
    }
}
fun main(){
    println(A.B())
    println(A().C())
    println(A.D)
    println(A.E)
    A.D.hello()
    A.hello()
}