GabrielBB GabrielBB - 1 month ago 4x
Java Question

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

i found this at

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())

My question is : Why do i have to Synchronize the list to iterate it if
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.


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.