Sealed class in kotlin

Hi Everyone,

Today we gonna discuss Sealed class in kotlin, Sealed is a new class type that introduced by kotlin.

Sealed class –  Sealed classes are used for representing restricted class hierarchies when a value can have one of the types from a limited set.

whereas a subclass of a sealed class can have multiple instances which can contain state.

there are few rules that make a class as a sealed

1. To declare a sealed class you have to put ‘sealed’ modifier before the name of the class.
2. A sealed class is abstract by itself, it cannot be instantiated directly and can have abstract members.
3. Sealed classes are not allowed to have non-private constructors
4. A sealed class can have subclasses, but all of them must be declared in the same file as the sealed class itself. but that classes which extend subclasses of a sealed class can be placed anywhere, not necessarily in the same file.

Let’s see the example as per kotlin reference doc

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

above we define sealed class Expr and there other classes that extend Expr

The key benefit of using sealed classes comes into play when you use them with WHEN expression. If its possible to verify that the statement covers all cases, you don’t need to add an else clause to the statement.if you remove any of the cases it will throw compile time error.

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

let’s create other class that extends Multi class, here we can place Multi class child into other files.

class ArrayMulti(val x: Expr, val y: Expr) : Multi(x,y)

Now you can also call ArrayMulti class in WHEN expression, but Expr immediate child(e.g Multi class) must be there.

let’s see an example.

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

Hope you like this. Thanks for reading 🙂