Is there a concurrent List in Java's JDK?

Solution 1:

ConcurrentLinkedQueue

If you don't care about having index-based access and just want the insertion-order-preserving characteristics of a List, you could consider a java.util.concurrent.ConcurrentLinkedQueue. Since it implements Iterable, once you've finished adding all the items, you can loop over the contents using the enhanced for syntax:

Queue<String> globalQueue = new ConcurrentLinkedQueue<String>();

//Multiple threads can safely call globalQueue.add()...

for (String href : globalQueue) {
    //do something with href
}

Solution 2:

There is a concurrent list implementation in java.util.concurrent. CopyOnWriteArrayList in particular.

Solution 3:

Disclaimer : This answer was published in 2011, before JDK 5, and before much advanced and optimal concurrent APIs. So, while the following will work, it is not the best option.

You can very well use Collections.synchronizedList(List) if all you need is simple invocation synchronization:

 List<Object> objList = Collections.synchronizedList(new ArrayList<Object>());

Solution 4:

Because the act of acquiring the position and getting the element from the given position naturally requires some locking (you can't have the list have structural changes between those two operations).

The very idea of a concurrent collection is that each operation on its own is atomic and can be done without explicit locking/synchronization.

Therefore getting the element at position n from a given List as an atomic operation doesn't make too much sense in a situation where concurrent access is anticipated.