Why am I getting an incompatible type error when using the List Iterator's next method but not-CodePudding

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,

CodePudding user response：

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);