hi friends,

I have little experience with hibernate.could you please tell me what is unidirectional and bidirectional assosiation and when it is required?

Well, 'unidirectional' and 'bidirectional' are not terms which are restricted to hibernate.
These are terms which belong to the common language of modeling,
(seehttp://edn.embarcadero.com/article/31863#classdiagrams)
In poor words:
-define the relation unidirectional if you need to navigate only in one direction (only either a.getB() or b.getA() exists)
-define the relation bidirectional if you need to navigate in both directions (a.getB() and b.getA() exists)