@Override
    public synchronized void put(final Bytes key, final byte[] value) {
        Objects.requireNonNull(key);
        synchronized (position) {
            if (value == null) {
                delete(key);
            } else {
                this.map.put(key, value);
            }
            StoreQueryUtils.updatePosition(position, context);
        }
    }
An in-memory LRU cache store based on HashSet and HashMap.
streams/src/main/java/org/apache/kafka/streams/state/internals/MemoryLRUCache.java