Hi , just wanted to understand what is the main difference between open and sealed class and also the use cases for its usage.

An open class can be subclassed by anyone (provided the constructor is accessible). A sealed class can only be subclassed by classes in the same file (or nested classes). This enables exhaustiveness-checking on

when

statements, since you know there will always be a fixed number of subtypes. Sealed classes are also

abstract

by default.

open

is not related to

sealed

!

open

is the opposite to Java’s

final

keywork -> no subclasses. While in Java the default for a class is to be open (hence you need

final

to declare your wish for the class to be non-extendable) the default for Kotlin classes is to be final (hence you need the

open

keyword to declare that the class is intended to be subclasses.

sealed

classes lie somewhere between enums and classes (and are sometimes called “enums with superpowers”). You can only subclass from within the same file, hence the compiler knows of all possible subclasses at compile time. Sealed classes are a non-trivial subject, so it’s better to check the official documentation for those: https://kotlinlang.org/docs/reference/sealed-classes.html

Thank you so much to both of you. It makes sense.

I didn’t get sealed classes at first, too. Now I often use them when I would use an Enum, but some enum values need additional data. That can best be understood with an example:

sealed class Escape {
    object Error : Escape() // Exception will be thrown if a character would need to be escaped
    object CStyle : Escape() // Escape using cstyle sequences like \t
    data class Character(val char: Char) : Escape() // Escape by prefixing with the given characters
}