Difference between open and override methods in Kotlin?

Question

open class Base {

    open fun v() {}

    fun nv() {}
}

class Derived() : Base() {

    override fun v() {}
}

This is an example. Can someone please

Answer 1

The open annotation on a class is the opposite of Java's final: it allows others to inherit from this class as by default all classes in Kotlin are final. [Source]

Only after declaring a class as open we can inherit that class.

A method can only be overridden if it is open in the base class. Annotation override signals the overriding of base method by inheriting class.

Answer 2

Yes, both open keywords are mandatory in your example.

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You have to distinguish between the use of open on classes and functions.

Class: You need the open keyword on a class if you want to inherit from it. By default all classes are final and can not be inherited from.

Function: On a function you need open to be able to override it. By default all functions are final and you cannot override them.

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Edit: Because I saw some confusion in the comments.

I have an internal abstract class which I can Inherit without any problem. I also can override it abstract methods as I please without declaring them as open

Abstract classes are meant to be inherited because you cannot instantiate them. in fact they are not just open by default, they cannot be final in the first place. final and abstract are not compatible. The same goes for abstract methods, they have to be overriden!

Answer 3

By default, the functions in Kotlin are defined as final. That means you cannot override them. If you remove the open from your function v() than you will get an error in your class Derived that the function v is final and cannot be overridden.

When you mark a function with open, it is not longer final and you can override it in derived classes.