@Override
    public synchronized void put(final K key,
        final V value) {
        if (value == null) {
            this.map.remove(key);
        } else {
            this.map.put(key, value);
        }
    }
This class is a generic version of the in-memory key-value store that is useful for testing when you
 need a basic KeyValueStore for arbitrary types and don't have/want to write a serde
streams/src/test/java/org/apache/kafka/test/GenericInMemoryKeyValueStore.java