@Override
    @Deprecated
    public <KR, VR> KStream<KR, VR> transform(final TransformerSupplier<? super K, ? super V, KeyValue<KR, VR>> transformerSupplier,
                                              final Named named,
                                              final String... stateStoreNames) {
        Objects.requireNonNull(transformerSupplier, "transformerSupplier can't be null");
        return flatTransform(new TransformerSupplierAdapter<>(transformerSupplier), named, stateStoreNames);
    }
Repartition a stream. This is required on join operations occurring after
an operation that changes the key, i.e, selectKey, map(..), flatMap(..).
streams/src/main/java/org/apache/kafka/streams/kstream/internals/KStreamImpl.java