void add(final Integer key, final long timestamp, final String value) {
            data.computeIfAbsent(key, k -> new HashMap<>());
            if (DELETE_VALUE_KEYWORD.equals(value)) {
                data.get(key).put(timestamp, Optional.empty());
            } else {
                data.get(key).put(timestamp, Optional.ofNullable(value));
            }
        }
In-memory copy of data put to versioned store, for verification purposes.
streams/src/test/java/org/apache/kafka/streams/integration/VersionedKeyValueStoreIntegrationTest.java