@Override
    public <K1, V1> KGroupedTable<K1, V1> groupBy(final KeyValueMapper<? super K, ? super V, KeyValue<K1, V1>> selector) {
        return groupBy(selector, Grouped.with(null, null));
    }
The implementation class of {@link KTable}.

@param <K> the key type
@param <S> the source's (parent's) value type
@param <V> the value type
streams/src/main/java/org/apache/kafka/streams/kstream/internals/KTableImpl.java