subclass.kt

public abstract class AbstractFoo<K, V> : kotlin.Map<K, V> {
    override operator fun get(key: K): V? {
        println("AbstractFoo")
        return null
    }

    override val size: Int
        get() = throw UnsupportedOperationException()

    override fun isEmpty(): Boolean {
        throw UnsupportedOperationException()
    }

    override fun containsKey(key: K): Boolean {
        throw UnsupportedOperationException()
    }

    override fun containsValue(value: V): Boolean {
        throw UnsupportedOperationException()
    }

    override val keys: Set<K>
        get() = throw UnsupportedOperationException()
    override val values: Collection<V>
        get() = throw UnsupportedOperationException()
    override val entries: Set<Map.Entry<K, V>>
        get() = throw UnsupportedOperationException()
}

public class IntFoo<E> : AbstractFoo<Int, E>() {
    override operator fun get(key: Int): E? {
        println("IntFoo")
        return null
    }
}