Hibernate and null fields

I was writing some simple slide generator. and in that process I write hbm.xml file and put this line in there.

<property name="photoSlideId" column="photo_slide_id" type="long" not-null="false" length="8" />

then I put needed variable into the class like this:
private long photoSlideId;

then I start to get photos list as null. it was impossible because it has to generate a exception or something in that situation. then I realized that this code was trying to assign null into the "long" type variable. it was impossible because in java we cannot assign null into the primitive types. then I changed class like this

private Long photoSlideId;

and its working like a charm.