Doubt in Collection.synchronizedSet(Set s)

Question

hi..
I am little bit confused with the below method..can anyone pls clear me..
In java.util.Collection class
public static Set synchronizedSet(Set s)
//Returns a synchronized (thread-safe) set backed by the specified set.
//In order to guarantee serial access, it is critical that all access to the
//backing set is accomplished through the returned set.
//It is imperative that the user manually synchronize on the returned set
//when iterating over it:
  Set s = Collections.synchronizedSet(new HashSet());
  synchronized(s) {
      Iterator i = s.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }when the set is already synchronized why shuld we again use synchronized block when iterarating it...
thnx,
mysha..

Answer 1

so when ur using synch block on tht set while
iterarting
synchronized(s){}
we r obtaining lock on the set also and even on this
block of code.....So while one therad is iterating
thru the synchronized block no other thread can
access the set 's' and also no other thread can
itearate thru this block..No, that's not correct. While you're in that block, no other code can obtain a lock on the Set. That's all. Any non-synchronized code can still modify the Set. So you need to synchronize all the code that can change the set, too. As someone mentioned if Iteration could take a long time, you may want to iterate over a copy, because it could impact performance. Then again, you need to make the program correct first, then fast.
and thus we can prevent
ConcuurentModificationException is this correct?I
didnt get ur reason for deadlocks though.
thnx,
mysha..