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 reason is quite simple:

You may read Objects from a Collection the same way as from Collection. But you can't add Objects to a Collection (The compiler forbids this) whereas to a Collection you can.

If after the release of Java 5 the compiler had translated every Collection to Collection, then previously written code would not compile anymore and thus would destroy the backward compatibility.