what is `open` keyword for fields in Kotlin?

In Kotlin open is the same as not final in Java for classes and methods.

What does open give me in the following class for the field marked as open?

@MappedSuperclass
abstract class BaseEntity() : Persistable<Long> {
     open var id: Long? = null
}

updated this is not duplicate of https://stackoverflow.com/questions/49024200/what-is-the-difference-between-open-and-public-in-kotlin

I am interested in open keyword for properties

updated

open class can be inherited.
open fun can be overridden
val property is final field in java

what about open property?

As you said, the open keyword allows you to override classes, when used in the class declaration. Accordingly, declaring a property as open, allows subclasses to override the property itself (e.g., redefine getter/setter). That keyword is required since in Kotlin everything is "final" by default, meaning that you can't override it (something similar to C#, if you have experience with that).

Note that your class is implicitly declared as open since it is abstract, hence you cannot create an instance of that class directly.

final method in Java: A method that cannot be overridden.

final class in Java: A class that cannot be extended.

Open classes and methods in Kotlin are equivalent to the opposite of final in Java, an open method is overridable and an open class is extendable in Kotlin.