In the code of page 4, it appears the line:

this.authors = new List<Author>();

but List in Java is an interface, not a concrete class. You should use ArrayList or LinkedList to instantiate the list, for example:

this.authors = new ArrayList<Author>();

And, if java 7 or higher is used, you can use the diamond operator to infer the type of the list.

this.authors = new ArrayList<>();

Juan Manuel

Thanks Juan. You are correct. I will fix that asap.

In the last PDF this is not yet corrected.