And the compiler knows exactly how many child class it has

How can that be helpful ?

when

cases. By knowing what are the possible children of a class, you can be sure that the when will be comprehensive

Exactly, it helps the compiler to warn you that you havn't handle all possible cases

Cheers guys

It’s useful when you wan’t enum-like behaviour, where some ‘values’ (child classes in the case of sealed classes) may have state, some may not have state and can be defined as

object

singletons, similar to standard enums.

I would say

selaed class

is a more powerful version of

Enum