What does the sealed modifier represent in Kotlin?

Question

I am little confused about the use of the sealed modifier.

What does it do?

Answer 1

The official docs cover this.

Sealed classes have restricted inheritance hierarchies: only classes that are declared inside them or are in the same file as them (since Kotlin 1.1) can be subclasses of a sealed class.

This can be useful when combined with when expressions, which can guarantee that their branches exhaustively check the possible subclasses of a sealed class.

Answer 2

This modifier is mainly use when you want to restrict the possibility of creating a subclass, it means all direct subclasses should be nested, this is an example:

sealed class Animal {
    class Cow(val name: String) : Animal()
}

//It generates a compilation error
class Horse : Animal() {
}

So, sealed classes can not have inheritors outside the class.

Answer 3

The other answers are good, but an important point I think that's worth adding: classes that extend subclasses of a sealed class can be placed anywhere, not necessarily in the same file. That's important to note, since a sealed class doesn't necessarily mean that an entire inheritance hierarchy will be in the same file unless every subclass is also sealed .

You can read more about this in the official docs for sealed classes.