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Sealed Classes

Sealed classes are used for representing restricted class hierarchies, when a value can have one of the types from a limited set, but cannot have any other type. They are, in a sense, an extension of enum classes: the set of values for an enum type is also restricted, but each enum constant exists only as a single instance, whereas a subclass of a sealed class can have multiple instances which can contain state.

To declare a sealed class, you put the sealed modifier before the name of the class. A sealed class can have subclasses, but all of them must be declared in the same file as the sealed class itself. (Before Kotlin 1.1, the rules were even more strict: classes had to be nested inside the declaration of the sealed class).

sealed class Expr
data class Const(val number: Double) : Expr()
data class Sum(val e1: Expr, val e2: Expr) : Expr()
object NotANumber : Expr()

(The example above uses one additional new feature of Kotlin 1.1: the possibility for data classes to extend other classes, including sealed classes.)

Note that classes which extend subclasses of a sealed class (indirect inheritors) can be placed anywhere, not necessarily in the same file.

The key benefit of using sealed classes comes into play when you use them in a when expression. If it's possible to verify that the statement covers all cases, you don't need to add an else clause to the statement. However, this works only if you use when as an expression (using the result) and not as a statement.

fun eval(expr: Expr): Double = when(expr) {
    is Const -> expr.number
    is Sum -> eval(expr.e1) + eval(expr.e2)
    NotANumber -> Double.NaN
    // the `else` clause is not required because we've covered all the cases
}