YailList is an object that can be only created or the elements inside are just readable.
That means you can only read elements from it.
Object list = yailList.toArray(); // converting YailList to array ArrayList<Object> objects = new ArrayList<>(Arrays.asList((yailList).toArray())); // Object arrayList
If you're trying to add an element then it's not possible. Instead, you can convert the normal list to