如何转换数组中的嵌套ArrayList？

import java.util.ArrayList;
import java.util.List;

public class ArrayConversion {
    public static void main(String[] args) {
        List<Object> nestedList = new ArrayList<>();
        nestedList.add(1);
        nestedList.add(new ArrayList<>());
        nestedList.add(new ArrayList<>());
        ((ArrayList)nestedList.get(1)).add(2);
        ((ArrayList)nestedList.get(1)).add(new ArrayList<>());
        ((ArrayList)nestedList.get(2)).add(3);
        ((ArrayList)nestedList.get(2)).add(new ArrayList<>());
        ((ArrayList)((ArrayList)nestedList.get(2)).get(1)).add(4);

        List<Object> flattenedList = flattenList(nestedList);
        System.out.println(flattenedList);
    }

    public static List<Object> flattenList(List<Object> nestedList) {
        List<Object> flattenedList = new ArrayList<>();
        for (Object element : nestedList) {
            if (element instanceof ArrayList) {
                flattenedList.addAll(flattenList((ArrayList) element));
            } else {
                flattenedList.add(element);
            }
        }
        return flattenedList;
    }
}