When used in combination with the when expression, sealed classes are like enum for classes:
sealed class Port
val name : String
get() = when (this) { //custom getter
is USB ->"this is USB"
is COM ->"this is COM"
}
class USB : Port()
class COM : Port()
USB u = USB
println (u.name) //this prints "this is USB"
Sealed classes are abstract, can have abstract members and define the classes that can inherit from them (USB and COM can inherit from Port)
miniguide 