What is the difference between 'open' and 'public' in Kotlin?

Question

I am new to Kotlin and I am confused between open and public keywords. Could anyone please tell me the difference between those keywords?

Answer 1

The open keyword means “open for extension“:

The open annotation on a class is the opposite of Java's final: it allows others to inherit from this class. By default, all classes in Kotlin are final, which corresponds to Effective Java, Item 17: Design and document for inheritance or else prohibit it.

You also need to be explicit about methods you want to make overridable, also marked with open:

open class Base {
    open fun v() {}
    fun nv() {}
}

The public keyword acts as a visibility modifier that can be applied on classes, functions etc. Note that public is the default if nothing else is specified explicitly:

If you do not specify any visibility modifier, public is used by default, which means that your declarations will be visible everywhere

Answer 2

class A { ... } in Java is equal to open class A { ... } in Kotlin.
final class B { ... } in Java is equal to class B { ...} in Kotlin.

It is not related with public.

In Kotlin, everything without access modifiers is public by default. You can explicitly say public in the definition, but it is not necessary in Kotlin.

So,

public class A { ... }

and

class A { ... }

are the same in Kotlin.

Answer 3

open is opposite to Final in java. If the class is not 'open', it can't be inherited.

class First{}
class Second:First(){}  // Not allowed. Since 'First' is Final(as in Java) by default. Unless marked "open" it can't be inherited 

Don't get confused with open and public. public is a visibility modifier

class Third{}  // By default this is public
private class Fourth{}
class Fifth{
    val third = Third() // No issues
    val fourth = Fourth() // Can't access because Fourth is private
}

Answer 4

I put here just for my memo, maybe useful for someone else :

open class in kotlin means that a class can be inherited because by default they are not:

 class Car{....}
 class Supercar:Car{....} : // give an error

 open Car2{....}
 class Supercar:Car2{....} : // ok

public class in Java is about the visibility of class (nothing to do with inheritance : unless a class in java is final, it can be inherited by default). In kotlin all the class are public by default.

open method in kotlin means that the method can be overridden, because by default they are not. Instead in Java all the methods can be overridden by default

The method of an open class cannot be overridden by default as usual (doesn't matter if the class is open), they must be declared that they can be overridden :

 class Car{....}
 class Supercar:Car{....} : // give an error

 open Car2{....}
 class Supercar:Car2{....} : // ok

for more details : https://kotlinlang.org/docs/reference/classes.html