What unit test to write for a class using generics in Java?

Taking the very specific example of the JpaDao class defined in this article:

public abstract class JpaDao<K, E> implements Dao<K, E> {
    protected Class<E> entityClass;

    @PersistenceContext
    protected EntityManager entityManager;

    public JpaDao() {
        ParameterizedType genericSuperclass = (ParameterizedType) getClass().getGenericSuperclass();
        this.entityClass = (Class<E>) genericSuperclass.getActualTypeArguments()[1];
    }

    public void persist(E entity) { entityManager.persist(entity); }

    public void remove(E entity) { entityManager.remove(entity); }

    public E findById(K id) { return entityManager.find(entityClass, id); }
}

would it be best to write unit tests for all the existing entities in the application (Order, Customer, Book, etc.), or would it be acceptable to write unit tests for just one entity, as hinted by this other question? Are there any best practice regarding unit testing java classes using generics?