java - Why can not I add an object of type Object into a List<?>?

Question

As far as I understand a List<?> is definds as a list of some specific , yet unknown type . So whatever is the parameter type of this List, it should be extending Object because in Java you can not have any type that doesn't extend Object. So why the following code doesn't get compiled ? How is it violating the the invariant of the listObj

    List<?> listObj = returnSomeList();
    listObj.add(new Object()); //Why does this not work ?

Answer 1

List<?> listObj can point to any type of List that can store different kind of objects.

How do you think, would it be safe to let it add any objects if for example

List<?> listObj = new ArrayList<String>();
listObj.add(new Object());