In Kotlin, are classes considered closed or open for inheritance by default?

In Kotlin, by default, all classes are final, meaning they cannot be inherited without explicit declaration.

To make a class inheritable, it must be declared with the open modifier:

open class BaseClass {
    // class body
}

class DerivedClass : BaseClass() {
    // subclass
}

This is done to enhance safety and prevent unintentional inheritance, which improves control over the class hierarchy.