wen I tried to explain the difference between

class

,

sealed class

and

open class

to a co-worker he asked: why is

sealed

not the default, given that developer has control on what goes into the same file anyway (i.e. why is

class

not the same as

sealed class

)? I did not have a good answer. can anyone help me out?

Because in many cases this is not you who extends your classes, but the users of your api.

A sealed class is open only in it's file, while class alone can't have any subclasses, even private ones

Also, sealed classes are a lot less common than regular classes in my experience

thanks for the answers. The argument that won me over was: "sealed classes are implicitly abstract". I of course do not want my classes be abstract by default, thus I see the necessity for another keyword to distinguish "cannot be instantiated but can be extended" from "cannot be instantiated but can be extended outside of the file where the class is defined".