ADT Stack with a doubly circular linked list
The class doesn't expose a method that let's users benefit from the circular property.
The enq, deq, front, empty methods could work just fine without the circular property.
As such, it's an unnecessary complexity.
If you want to use a circular linked list,
then expose some methods that need or benefit from the circular property.

The class QueueCircularDLL&lt;T&gt; implements the ADTQueue&lt;T&gt; interface,
but it's not clear which methods belong to the interface.
It's strongly recommended to make that clear using the @Override annotation.

In front, instead of returning null when the list is empty,
a more common practice is to throw java.util.NoSuchElementException,
as done by the various linked list and queue implementations in the JDK.