It is checked here:
https://github.com/julienledem/summingbird/blob/d88553b193c24b39175bb19e3bab46deb5006997/summingbird-core/src/main/java/com/twitter/summingbird/javaapi/JKeyedProducer.java#L28
The platform will provide the right types. Those parameters are here to be carried to the Producer.