How to clone a synchronized Collection?

Question

Imagine a synchronized Collection:

Set s = Collections.synchronizedSet(new HashSet())

What's the best approach to clone this Collection?

It's prefered that the cloning doesn't need any synchronization on the original Collection but required that iterating over the cloned Collection does not need any synchronization on the original Collection.

Answer 1

Use a copy-constructor inside a synchronized block:

synchronized (s) {
    Set newSet = new HashSet(s); //preferably use generics
}

If you need the copy to be synchronized as well, then use Collections.synchronizedSet(..) again.

As per Peter's comment - you'll need to do this in a synchronized block on the original set. The documentation of synchronizedSet is explicit about this:

It is imperative that the user manually synchronize on the returned set when iterating over it