@Override
    public synchronized byte[] putIfAbsent(final Bytes key, final byte[] value) {
        Objects.requireNonNull(key);
        final byte[] originalValue = get(key);
        if (originalValue == null) {
            put(key, value);
        }
        return originalValue;
    }
An in-memory LRU cache store based on HashSet and HashMap.
streams/src/main/java/org/apache/kafka/streams/state/internals/MemoryLRUCache.java