Why is Collection not simply treated as Collection<?>

Question

Consider the following API method taken from Shiro\'s org.apache.shiro.subject.PrincipalCollection interface but probably present in other libraries as well:

Answer 1

A use-case that I can think of as to why Collection is not considered as Collection is let say we have a instance of ArrayList

Now if the instance is of type ArrayList or ArrayList or ArrayList, you can add that type only(type checking). ArrayList is not equivalent to ArrayList