Kotlin: "abstract member cannot be accessed directly" when using delegation ("by" keyword)

Question

This is the sample code:

class PurchaseHistory: MutableList<PurchaseInfo> by mutableListOf() {
    override fun add(element: PurchaseInfo): Boolean {
        // ... some code
        return super.add(element)
    }
}

However, I am getting abstract member cannot be accessed directly. When I use the original method from outside the class, I don't encounter any issues, but why can't I do the same thing from inside of it?

Answer 1

You get this error because the add method you're trying to call is not really in PurchaseHistory's super class (because it doesn't have a superclass), and thus the error tells you that you cannot just call an interface (abstract) method using super.

To do what you want, you can keep a handle to the object you are delegating to. For instance, you can store it as a property:

class PurchaseHistory(
    private val backingList: MutableList<PurchaseInfo> = mutableListOf()
): MutableList<PurchaseInfo> by backingList {
    override fun add(element: PurchaseInfo): Boolean {
        // ... some code
        return backingList.add(element)
    }
}

Another option is to directly extend an implementation of MutableList (such as ArrayList) instead of implementing by delegation, but that might not be an option for you.