src/main/java/org/jivesoftware/smack/util/collections/AbstractReferenceMap.java
protected void purge() {
        Reference ref = queue.poll();
        while (ref != null) {
            purge(ref);
            ref = queue.poll();
        }
    }
protected void purge() {
        Reference ref = queue.poll();
        while (ref != null) {
            purge(ref);
            ref = queue.poll();
        }
    }
Purges stale mappings from this map.
<p/>
Note that this method is not synchronized! Special care must be taken if,
for instance, you want stale mappings to be removed on a periodic basis
by some background thread.