Synchronized collection

Question

Since c is already synchronzied collection, and it's thus thread safe. But why do we have to use synchronized(c) again for the iteration? Really confused. Thanks.

" It is imperative that the user manually synchronize on the returned collection when iterating over it:

Collection c = Collections.synchronizedCollection(myCollection);
 ...
 synchronized(c) {
     Iterator i = c.iterator(); // Must be in the synchronized block
     while (i.hasNext()) {
         foo(i.next());
     }
}

Failure to follow this advice may result in non-deterministic behavior. " http://docs.oracle.com/javase/6/docs/api/java/util/Collections.

Answer 1

The most any synchronized collection implementation could possibly do is to guarantee that each individual method call is synchronized. But iteration necessarily involves multiple separate method calls, so you, the user of the synchronized collection, have to synchronize on the whole iteration yourself.

For example, if you didn't synchronize on c, the contents of the collection could change between i.hasNext() and i.next() -- it could even go from having elements to having no more elements, in which case i.next() would fail.