private ValueAndTimestamp<KeyValue<K1, V1>> mapValue(final K key, final ValueAndTimestamp<V> valueAndTimestamp) {
            return ValueAndTimestamp.make(
                mapper.apply(key, getValueOrNull(valueAndTimestamp)),
                valueAndTimestamp == null ? context.timestamp() : valueAndTimestamp.timestamp()
            );
        }
@throws StreamsException if key is null
streams/src/main/java/org/apache/kafka/streams/kstream/internals/KTableRepartitionMap.java