In your example, it is because you can’t have a List of a primitive type. In other words, List<int> is not possible.
You can, however, have a List<Integer> using the Integer class that wraps the int primitive. Convert your array to a List with the Arrays.asList utility method.
Integer[] spam = new Integer[] { 1, 2, 3 };
List<Integer> list = Arrays.asList(spam);