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

The major difference between raw type and unbounded wildcard is that the latter is type safe, that is, on a compile level, it checks whether the items in the collection are of the same type. Compiler won't allow you to add string and integer to the collection of wildcard type, but it will allow you to do this:

List raw = new ArrayList();
raw.add("");
raw.add(1);

Actually, in case of unbounded wildcard collections (List wildcard = new ArrayList()), you can't add anything at all to the list but null (from Oracle docs):

Since we don't know what the element type of c stands for, we cannot add objects to it. The add() method takes arguments of type E, the element type of the collection. When the actual type parameter is ?, it stands for some unknown type. Any parameter we pass to add would have to be a subtype of this unknown type. Since we don't know what type that is, we cannot pass anything in. The sole exception is null, which is a member of every type.