@Override
    public <VOut> CogroupedKStream<K, VOut> cogroup(final Aggregator<? super K, ? super V, VOut> aggregator) {
        Objects.requireNonNull(aggregator, "aggregator can't be null");
        return new CogroupedKStreamImpl<K, VOut>(name, subTopologySourceNodes, graphNode, builder)
            .cogroup(this, aggregator);
    }
@Test
    public void shouldNotHaveNullAggregatorOnCogroup() {
        assertThrows(NullPointerException.class, () ->  groupedStream.cogroup(null));
    }