public void put(final K key, final V value, final long timestamp) {
        if (!data.containsKey(timestamp)) {
            data.put(timestamp, new TreeMap<>());
        }
        data.get(timestamp).put(key, value);
    }
A very simple window store stub for testing purposes.
streams/src/test/java/org/apache/kafka/streams/state/internals/ReadOnlyWindowStoreStub.java