java - Why do I need to synchronize a list returned by Collections.synchronizedList

Question

i found this at dos.oracle.com

public static List synchronizedList(List list)

Returns a synchronized (thread-safe) list backed by the specified list. In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list. It is imperative that the user manually synchronize on the returned list when iterating over it:

  List list = Collections.synchronizedList(new ArrayList());
      ...
  synchronized(list) {
      Iterator i = list.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }

My question is : Why do i have to Synchronize the list to iterate it if Collections.synchronizedList(); is supposed to return an already synchronized list ?

Im just accesing the list in two threads: One Thread just add and the other thread to get and delete. What other classes you recommend to use for this scenario ?

Thanks for reading.

Answer 1

The list being synchronized only means that add, remove etc. operations are synchronized and therefore atomic. Iteration however is not and if a thread adds while another is iterating, you could get a ConcurrentModificationException.

By manually synchronizing your iteration block, you ensure that the list is not modified while iterating.

One alternative is to use a CopyOnWriteArrayList which provides an iterator that iterates over the list as it was known when the iteration started, regardless of subsequent modifications. That collection is however not very efficient if you need to change the content of the list very often.