#fixed code
                                InternalOakMap(K minKey,
                   OakSerializer<K> keySerializer,
                   OakSerializer<V> valueSerializer,
                   OakComparator<K> oakComparator,
                   MemoryManager memoryManager,
                   int chunkMaxItems) {

        this.size = new AtomicInteger(0);
        this.memoryManager = memoryManager;

        this.keySerializer = keySerializer;
        this.valueSerializer = valueSerializer;

        this.comparator = oakComparator;

        this.minKey = ByteBuffer.allocate(this.keySerializer.calculateSize(minKey));
        this.minKey.position(0);
        this.keySerializer.serialize(minKey, this.minKey);

        // This is a trick for letting us search through the skiplist using both serialized and unserialized keys.
        // Might be nicer to replace it with a proper visitor
        Comparator<Object> mixedKeyComparator = (o1, o2) -> {
            if (o1 instanceof ByteBuffer) {
                if (o2 instanceof ByteBuffer) {
                    return oakComparator.compareSerializedKeys((ByteBuffer) o1, (ByteBuffer) o2);
                } else {
                    // Note the inversion of arguments, hence sign flip
                    return (-1) * oakComparator.compareKeyAndSerializedKey((K) o2, (ByteBuffer) o1);
                }
            } else {
                if (o2 instanceof ByteBuffer) {
                    return oakComparator.compareKeyAndSerializedKey((K) o1, (ByteBuffer) o2);
                } else {
                    return oakComparator.compareKeys((K) o1, (K) o2);
                }
            }
        };
        this.skiplist = new ConcurrentSkipListMap<>(mixedKeyComparator);

        Chunk<K, V> head = new Chunk<>(this.minKey, null, this.comparator, memoryManager, chunkMaxItems,
                this.size, keySerializer, valueSerializer);
        this.skiplist.put(head.minKey, head);    // add first chunk (head) into skiplist
        this.head = new AtomicReference<>(head);
    }
#vulnerable code 
                                Result<V> putIfAbsent(K key, V value, Function<ByteBuffer, V> transformer) {
        if (key == null || value == null) {
            throw new NullPointerException();
        }

        Chunk<K, V> c = findChunk(key); // find chunk matching key
        Chunk.LookUp lookUp = c.lookUp(key);
        if (lookUp != null && lookUp.valueSlice != null) {
            if (transformer == null) {
                return Result.withFlag(false);
            }
            AbstractMap.SimpleEntry<ValueUtils.ValueResult, V> res = ValueUtils.transform(lookUp.valueSlice,
                    transformer);
            if (res.getKey() == SUCCESS) {
                return Result.withValue(res.getValue());
            }
            return putIfAbsent(key, value, transformer);
        }

        // if chunk is frozen or infant, we can't add to it
        // we need to help rebalancer first, then proceed
        Chunk.State state = c.state();
        if (state == Chunk.State.INFANT) {
            // the infant is already connected so rebalancer won't add this put
            rebalance(c.creator());
            return putIfAbsent(key, value, transformer);
        }
        if (state == Chunk.State.FROZEN || state == Chunk.State.RELEASED) {
            rebalance(c);
            return putIfAbsent(key, value, transformer);
        }


        int ei;
        long oldReference = DELETED_VALUE;

        // Is there already an entry associated with this key?
        if (lookUp != null) {
            // There's an entry for this key, but it isn't linked to any value (in which case valueReference is
            // DELETED_VALUE)
            // or it's linked to a deleted value that is referenced by valueReference (a valid one)
            ei = lookUp.entryIndex;
            assert ei > 0;
            oldReference = lookUp.valueReference;
        } else {
            ei = c.allocateEntryAndKey(key);
            if (ei == -1) {
                rebalance(c);
                return putIfAbsent(key, value, transformer);
            }
            int prevEi = c.linkEntry(ei, key);
            if (prevEi != ei) {
                // some other thread linked its entry with the same key.
                oldReference = c.getValueReference(prevEi);
                if (oldReference != DELETED_VALUE) {
                    if (transformer == null) {
                        return Result.withFlag(false);
                    }
                    AbstractMap.SimpleEntry<ValueUtils.ValueResult, V> res =
                            ValueUtils.transform(c.buildValueSlice(oldReference), transformer);
                    if (res.getKey() == SUCCESS) {
                        return Result.withValue(res.getValue());
                    }
                    return putIfAbsent(key, value, transformer);
                } else {
                    // both threads compete for the put
                    ei = prevEi;
                }
            }
        }

        long newValueReference = c.writeValue(value); // write value in place

        Chunk.OpData opData = new Chunk.OpData(Operation.PUT_IF_ABSENT, ei, newValueReference, oldReference, null);

        // publish put
        if (!c.publish()) {
            c.releaseValue(newValueReference);
            rebalance(c);
            return putIfAbsent(key, value, transformer);
        }

        if (!finishAfterPublishing(opData, c)) {
            c.releaseValue(newValueReference);
            return putIfAbsent(key, value, transformer);
        }

        return transformer != null ? Result.withValue(null) : Result.withFlag(true);
    } 
                            #location 83 
                            #vulnerability type NULL_DEREFERENCE