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
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