joshua-r2
10/31/2017 - 6:02 PM

Sealed Classes

Sealed classes have the ability to restrict subclass creation; it has hierarchy which is is fixed. This comes in handy for having a defined number of Subclass Types. In a "when" statement, we typically need a default branch ("else"). In the example below, our subclasses are products. What if we added a new product besides refrigerators and washing machines? The compiler wouldn't know and would always default to our else branch, making an unwanted bug.

NoSealedClass
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Products
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//prefix the class with the sealed keyword to indicate the class is sealed
sealed class Product(val modelNo: String) {

    constructor() : this("")
}


data class WashingMachine(val capacity: Int, val model: String): Product(model)
data class Refrigerator(val cubicFeet:  Long, val model: String): Product(model)


//A great use case for sealed classes is the when statement. It keeps the options limited (no default branch needed)
// to a certain number of subclasses
fun getExcitedAboutProduct(product: Product): String =
        when(product)
        {
            is WashingMachine -> "I mean, does it work quietly???"
            is Refrigerator -> "Oh snap, it has a touchscreen!?!"
        }


/*
  Adding a data new dataclass that extends Product would produce a compiler error, because
  our "when" statement does not address all children of the sealed class
*/

fun main(args: Array<String>)
{
    println(getExcitedAboutProduct(WashingMachine(33, "ML24A")))
}