@Override
    public <K1> KStream<K1, V> toStream(final KeyValueMapper<? super K, ? super V, ? extends K1> mapper,
                                        final Named named) {
        return toStream(named).selectKey(mapper);
    }
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