Add header or text at start of next section group in RecyclerView

Question

I want to add a section header or something to state its a new section but I just can't get it right. The testSection.setHeader() method I commented out requires an item called Header : Group.

This is the code for the recycler view:

private fun updateRecyclerViewTest(items: List<Item>) {
    fun init() {
        recycler_view_test.apply {
            layoutManager = LinearLayoutManager(this@EditModActivity)
            adapter = GroupAdapter<ViewHolder>().apply {
                testSection = Section(items)
                assSection = Section(items)
                //pracSection = Section(items)
                add(testSection)
                add(assSection)
                //add(pracSection)
                setOnItemClickListener(onItemClick)
            }
        }
        shouldInitRecyclerView = false
    }
    if (shouldInitRecyclerView)
        init()
    else
    {

        testSection.update(items.subList(0,3))
        //testSection.setHeader()
        assSection.update(items.subList(4,6))
        //pracSection.update(items.subList(6,9))
    }
}

This is the current output