为什么在使用 List Iterator 的 next 方法时会出现类型不兼容的错误，而在使用 List 的 get 方法时却不会？

Question

public static void main(String args[]) 
{
    ArrayList<String> names = new ArrayList<String>();
    names.add("Brett");
    
    ArrayList<String> names2 = new ArrayList<String>();
    names2.add("John");
    
    append(names, names2);
    
}

public static <E> void append(List<E> list1, List<E> list2)
{
    Iterator list2Iterator = list2.listIterator();
    while(list2Iterator.hasNext())
    {
        list1.add(list2Iterator.next());
    }
}

I was asked to append the elements of one list to another. I used the list2's iterator to retrieve each element, and passed them to list1's add method. But I got this error: incompatible types: Object cannot be converted to E. However, if I modify the append method:

public static <E> void append(List<E> list1, List<E> list2)
{
    for (int i = 0; i < list2.size(); i++)
        list1.add(list2.get(i));
}

it works just fine. Both the list iterator and the get method return the same type, but only the latter works. I'm sure the reason is simple, I just haven't figured it out so far.

Thanks,

Answer 1

The problem is that in your code you are loosing the type parameter when you obtain the list iterator.

public static <E> void append(List<E> list1, List<E> list2) {
    Iterator<E> list2Iterator = list2.listIterator();
    while(list2Iterator.hasNext()) {
        list1.add(list2Iterator.next());
    }
}

But you should avoid manually adding all elements of a collections 1 by 1 to another collection. Here is a better solution:

list1.addAll(list2);