@Override
    public V fetch(final K key, final long time) {
        final Map<K, V> kvMap = data.get(time);
        if (kvMap != null) {
            return kvMap.get(key);
        } else {
            return null;
        }
    }
A very simple window store stub for testing purposes.
streams/src/test/java/org/apache/kafka/streams/state/internals/ReadOnlyWindowStoreStub.java