Sealed class in Kotlin

To constrain a class hierarchy, we have to declare the superclass sealed. Without the constraint, our code will look like this:

open class Transport

data class Train(
  val line: String
): Transport()

data class Bus(
  val number: String
): Transport()

fun travel(transport: Transport) =
  when (transport) {
    is Train -> "Train ${transport.line}"
    is Bus -> "Bus ${transport.number}"
    else -> "Unknown transport"
  }

If you are looking at travel() method, you will notice there is a potential trouble. Imagine if we have another Transport type, the travel() method will not tell you that you need to modify it to detect for new Transport type. If you have a lot of Transport types, then it could become a maintenance nightmare. This whole situation can be improved using sealed keyword. When defining Transport, replace open class with sealed class.

sealed class Transport

data class Train(
  val line: String
): Transport()

data class Bus(
  val number: String
): Transport()

fun travel(transport: Transport) =
  when (transport) {
    is Train -> "Train ${transport.line}"
    is Bus -> "Bus ${transport.number}"
  }

all direct subclasses of a sealed class must be located in the same file as the base class. Although Kotlin forces you to exhaustively check all possible types in a when expression, the when in travel() no longer requires an else branch.

Subclasses

When a class is sealed, you can easily iterate through its subclasses:

sealed class Android
class Samsung : Android()
class Huawei : Android()
open class LG : Android()
class LG_One : LG()


fun main() {
  Android::class.sealedSubclasses.map { it.simpleName }
  // Samsung, Huawei, LG
}

Android::class will produce a class object and you can access properties and member functions of that class object to discover information. One of the properties of class objects is sealedSubclasses, which expects that Android is a sealed class (otherwise it produces an empty list).

sealedSubclasses uses reflection, which requires that the dependency kotlin-reflection.jar be in the classpath. Reflection is a way to dynamically discover and use characteristics of a class. sealedSubclasses can be an important tool when building polymorphic systems. It can ensure that new classes will automatically be included in all appropriate operations. Because it discovers the subclasses at runtime, however, it may have a performance impact on your system.