JPA+Tutorial+4+-+Introduction

So far, every query we've used has returned a single kind of result. In this tutorial we change things up a bit by moving from only using Books in our Library to working with Books and Dvd's, both of which inherit from **Resource**. When we search for a **Resource** we might get back Books, Dvd's, or both. This makes our queries polymorphic.

We pick up at the end of JPA Tutorial 3 - A Mini Application and perform two major transformations:
 * 1) Introduce the Resource class and make the appropriate refactorings
 * 2) Introduce a new type, Dvd