I think iterator method could have also handled synchronization in the same fashion as above method
No, it absolutely couldn't.
The iterator has no control over what your code does between calls to the individual methods on it. That's the point. Your iteration code will call hasNext()
and next()
repeatedly, and synchronization during those calls is feasible but irrelevant - what's important is that no other code tries to modify the list across the whole time you're iterating.
So imagine a timeline of:
t = 0: call iterator()
t = 1: call hasNext()
t = 2: call next()
// Do lots of work with the returned item
t = 10: call hasNext()
The iterator can't synchronize between the end of the call to next()
at t=2 and the call to hasNext()
at t=10. So if another thread tries to (say) add an item to the list at t=7, how is the iterator meant to stop it from doing so?
This is the overall problem with synchronized collections: each individual operation is synchronized, whereas typically you want a whole chunky operation to be synchronized.