final Object newKeyReference(K key, ReferenceType keyType,
                                     ReferenceQueue<Object> refQueue) {
            if (keyType == ReferenceType.WEAK)
                return new WeakKeyReference<K>(key, hash, refQueue);
            if (keyType == ReferenceType.SOFT)
                return new SoftKeyReference<K>(key, hash, refQueue);

            return key;
        }
ConcurrentReferenceHashMap list entry. Note that this is never exported
out as a user-visible Map.Entry.

Because the value field is volatile, not final, it is legal wrt
the Java Memory Model for an unsynchronized reader to see null
instead of initial value when read via a data race.  Although a
reordering leading to this is not likely to ever actually
occur, the Segment.readValueUnderLock method is used as a
backup in case a null (pre-initialized) value is ever seen in
an unsynchronized access method.
commons-profiler/src/main/java/com/navercorp/pinpoint/common/profiler/concurrent/jsr166/ConcurrentReferenceHashMap.java