the list used as a lock is an instance of class ArrayList. According to specification ArrayList implementation, an Arraylist is not synchroized, ie if multiple threads start accessing and or modifying the arraylist the state(data) of arraylist may get corrupted. So the instance "list" is used as lock so that block of code is synchronized and only one thread can access the list at any given point of time. do rate the answer or correct me if i am wrong. thanx Shreyas Rane