CODEWITHSUNDEEP
kotlin
Matthew Layton
Matthew Layton

Reputation: 42260

Does Kotlin Support Member Sealed Overrides?

In Kotlin we can modify a class with the sealed modifier to denote that the class can only be extended within the same module; or from the docs:

Sealed classes and interfaces represent restricted class hierarchies that provide more control over inheritance. All subclasses of a sealed class are known at compile time. No other subclasses may appear after a module with the sealed class is compiled. For example, third-party clients can't extend your sealed class in their code. Thus, each instance of a sealed class has a type from a limited set that is known when this class is compiled.

What I want to know is, can the same or similar behaviour be applied to class members; for example, consider the following code. All of the classes and interfaces exist in the same module:

// I don't want to seal this because it should be implementable beyond this module.
interface Hashable {
    val hash: Hash
}

// I don't want to seal this because it should be extensible beyond this module.
abstract class Base : Hashable {
    final override val hash: Hash get() = hashOf(...)
}

open class Derived : Base {

    // This doesn't work because it's final in the base class.
    final override val hash: Hash get() = hashOf(...)
}

What I essentially want to express is, "I (the developer) determine how a Base should create it's hash, until I (the developer) state otherwise in a derived class. Nobody else outside of this module has the ability to alter how each extension or derivative of Base may create its hash."

Is this possible?

Upvotes: 0

Views: 645

Answers (1)

broot
broot

Reputation: 28332

You can create additional property with internal visibility:

abstract class Base : Hashable {
    final override val hash: Hash get() = _hash
    internal open val _hash: Hash get() = hashOf(...)
}

open class Derived : Base() {
    override val _hash: Hash get() = hashOf(...)
}

Upvotes: 1

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