How to update individual record inside cached list object??

hi, i am storing in cache list of objects with a particular key name to that list .So while searching with that key name i am able to get all the objects as a list successfully. But if i have to update a single object record in that list how can i get it from cache and get it updated.
Eg i have car and bike as two different class.While storing in cache ,with key name as 'car' and 'bike' ,i store list of 10 car objects and 10 bike objects.
But to update a single car object within that list..how to retrive it from cache..since in cache the entire list is stored as key 'car'.