@SuppressWarnings("unchecked")
        void removeStale() {
            WeakKeyReference<K> ref;
            while ((ref = (WeakKeyReference<K>) refQueue.poll()) != null) {
                remove(ref, ref.hash, null, true);
            }
        }
Remove; match on key only if value null, else match both.
commons-profiler/src/main/java/com/navercorp/pinpoint/common/profiler/concurrent/jsr166/ConcurrentWeakHashMap.java