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javakotlindecoratordelegation

Can I extend, in Java, a Kotlin delegating class?

I'm trying to extend, in Java, a Kotlin delegating class and get the following error:

Cannot inherit from final 'Derived'

See code below.

What I'm trying to do is decorate a method of a class.

Any idea why Kotlin defined Derived as final? Is there a way for Derived to not be final so I can inherit it?

Java:

new Derived(new BaseImpl(10)) { // Getting the error on this line: `Cannot inherit from final 'Derived'`
};

Kotlin:

interface Base {
    fun print()
}

class BaseImpl(val x: Int) : Base {
    override fun print() { print(x) }
}

class Derived(b: Base) : Base by b

* Example from here: https://kotlinlang.org/docs/reference/delegation.html

Solution

  • Kotlin classes are final in by default. Mark the class as open to extend it (whether from Kotlin or from Java):

    open class Derived(b: Base) : Base by b

    The fact this is a delegating class should have no impact on Java interop (although I haven't tried it).