<？的必要性？super T>在Collections.copy中输入

Question

Collections.copy方法签名是,

public static <T> void copy(List<? super T> dest, List<? extends T> src)

只是想知道dest类型是否必须<? super T>而不仅仅是<T>？

public static void main(String[] args)
{
  simpleCopy(new ArrayList<Number>(), new ArrayList<Number>());
  simpleCopy(new ArrayList<Number>(), new ArrayList<Integer>());
}

public static <T> void simpleCopy(Collection<T> dest, Collection<?  extends T> src)
{
   for (T element : src)
     dest.add(element);
}