Right use of covariant and contravariance in kotlin for treeview

Question

Hello stackoverflow communtiy,

I wrote a method using generic types. And I have the strange feeling that it's a little bit to complex. On the internet I found the concept of contra variance and variance for generic types. But my attempts to do it right were unsuccessful. Is there a way to get rid of the second (redundant) generic Type K?

fun <T, K> add(item: TreeItem<K>, startParent: TreeItem<T>, levelIndices: List<Int>) where K : T {
    var currentParent = startParent
    for ((counter, levelIndex) in levelIndices.withIndex()) {
        if (counter == levelIndices.size - 1) {
            @Suppress("UNCHECKED_CAST")
            currentParent.children.add(levelIndices.last(), item as TreeItem<T>)
            break
        }
        val positionEntryController = currentParent.children[levelIndex].value
        if (positionEntryController is PositionHeaderController) {
            currentParent = currentParent.children[levelIndex]
        } else {
            throw NotImplementedError()
        }
    }
}

The crucial line is under @Suppress("UNCHECKED_CAST"), where I have to cast explicitly.

score 0 · Answer 1 · answered Jan 14 '20 at 12:29

0

I think this should allow you to remove the cast

fun <T> add(item: TreeItem<in T>, startParent: TreeItem<T>, levelIndices: List<Int>) {

answered Jan 14 '20 at 12:29

lyrjie

141
1
5

Unfortunately this does not work. ```add(TreeItem(SubClass), TreeItem(Class), ...)``` does not work. – Christian Jan 14 '20 at 15:34
It should be `out T` instead of `in T`, but I don't see why it would avoid the cast. – Alexey Romanov Jan 15 '20 at 14:08

Right use of covariant and contravariance in kotlin for treeview

1 Answers1