public interface KGroupedStream <K,V> {

    KTable <K,Long> count ();
    KTable <K,V> reduce (Reducer <V> reducer);

    <VR> KTable <K,VR> aggregate (
        Initializer <VR> initializer,
        Aggregator <? super K, ? super V, VR> aggregator);

    <W extends Window> TimeWindowedKStream <K,V> windowedBy (Windows<W> windows);

    ...
}