Practical example of sealed classes in Android Development

Question

I'm newbie of Kotlin. I'm learning sealed classes and I don't understand how could I use it in Android development. Can you give me an example?

Answer 1

It's useful when you want to define a closed list of subclasses.

You can use possibility for data classes to extend sealed classes . Example:

 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
 }

Declaration:

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