Java Collection<Object> or Collection<?>

Question

I try use List instead of List

List<?> list = new ArrayList<Integer>();
...
list.add(1); //compile error

What i do wrong and how cast new value to Integer? Maybe i shoud use List in my project?

Answer 1

List<?> means a list of some type, but we don't know which type. You can only put objects of the right type into a list - but since you don't know the type, you in effect can't put anything in such a list (other than null).

There is no way around this, other than declaring your variable as List<Integer>.

Answer 2

List<Integer> list = new ArrayList<Integer>();

The generic type always has to be the same.