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